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THE f ORMAN SERIES IN 





M, 

LANGUAGE. 


A Complete Course in German. 

st m 

By JAMES H. WORMAN, A.M. 

EMBRACING 

ELEMENTARY GERMAN (GRAIMIVLAR, 

COMPLETE GERMAN CGRAMllVXAR, 

COLLEGIATE GERMAN READER, 
ELEMENTARY GHERMAN READER, 
GERMAN COPY-BOOKS, GERMAN ECHO. 

HISTORY' OE GERMAN LITERATURE, 

GERMAN AND ENGLISH LEXICON, 

I. THE GERMAN GRAMMARS of Worman are widely preferred on ac¬ 
count of their clear, explicit method (on the conversation plan), introducing a system 
of analogy and comparison with the learners’ own language and others commonly 
studied. 

The arts of speaking, of understanding the spoken language, and of correct pronun¬ 
ciation, are treated with great success. 

The new classifications of nouns and of irregular verbs are of great value to the 
pupil. The use of heavy type to indicate etymological changes, is new. The Vocabu¬ 
lary is synonymical —also a new feature. 

II. WORMAN’S GERMAN REARER contains progressive selections 
from a wide range of the very best German authors, including three complete plays, 
which are usually purchased in separate form for advanced students who have com¬ 
pleted the ordinary Reader. 

It has Biographies of eminent authors. Notes after the text, References to all Ger¬ 
man Grammars in common use, and an adequate Vocabulary; also, Exercises for 
translation into the German. 

III. WORMAN’S GERMAN ECHO {Deutsches Echo) is entirely a new 
thing in this country. , It presents familiar colloquial exercises without translation, 
and will teach fluent conversation in a few months of diligent study. 

No other method will ever make the student at hoine in a foreign language. By this 
he thinks in, as well as speaks it. For the time being he is a German through and 
through. The laborious process of translating his thoughts no longer impedes free 
unembarrassed utterance. 


WOEMAH’S COMPLETE FBENCE COUESE 

IS INAUGURATED BY 

E’EOEIIO IDE 3? IFL I S 3 

Or “French Echo;” on a plan identical with the German Echo described above. 
This will be followed in due course by the other volumes of 

THE FRENCH SERIES, 
yiz. : 

A COMFLETE GRAMMAR , I A FRENCH REARER , 

AN ELEMENTARY GRAMMAR, \ A FRENCH LEXICON, 

A HISTORY OF FRENCH LITERATURE. 

-.>♦ » «- 

tWORMAN’S WORKS 

are adopted as fast as published by many of the best institutions of the country. In 
completeness, adaptation, and homogeneity for consistent courses of instruction, they 

are simply 

UNRIVALED, 







\ 






V 


































■*. 















































'■ • 















































High-pressure Steam 














































































































































































































































































































































































































































































FOURTEEN WEEKS 


T_N 


NATUIiAL PHILOSOPHY, 


\ BY 

& 

J. DORMAN STEELE, Pn.D., 

u 

AUTHOB OV TUB FOURTEBN-WEEKS RER1K8 IN PHYSIOLOGY, CHEMISTRY, 
ASTRONOMY, AND GEOLOGY. 


“ The worts of God are fair for naught, 

Unless our eyes, in seeing. 

See hidden in the thing the thought 
That animates its being." —Tilton. 


A. 


S. BARNES & COMPANY, 

NEW YORK, CHICAGO and NEW ORLEANS. 

1878. 


9 } 

> 

) > 

> > 


J > 





*> 


"5. 


TIJE FOURTEEN WEEKS’ COURSES 

IN 

NATURAL SCIENCE, 

BY 

J. DORMAN STEELE, A.M., Ph.D. 


Fourteen Weeks iq Natural Philosophy, 
Fourteen Weeks iij Chemistry, (2 Editions) 
Fourteen Weeks in Descriptive ^stroijomy, 
Fourteen Weeks in Popular Geology, 

Fourteen Weeks in Hunjan Physiology, 

A Key, containing Answers to the Questions 
and Problems in Steele’s 14 Weeks’Courses, 


4 HISTORICAL SERIES, 

on the plan of Steele’s 14 Weeks in the Sciences, 


A Brief History of the United States, 
A Brief tyistory of France, 


The publishers of this volume will send either of the above by 
mail, post-paid, on receipt of the price. 

The same publishers also offer the following standard scientific 
works, being more extended or difficult treatises than those of 
Prof. Steele, though still of Academic grade. 

Peck’s Gaijot’s Natural Philosophy, 

Porter’s Principles of Chemistry, 

Jarvis’ Physiology aijd Laws of tyealth, 

Wood’s Botanist and Florist, 

Chambers’ Elements of Zoology, 

McIntyre’s Astronomy and the Globes, 

Page’s Elements of Geology, 

Address A. S. BARNES & CO., 

Educational Publishers, 

NEW YORK OR CHICAGO. 


Entered according to Act of Congress, in the year 1869, by 
A. S. BARNES & CO., 

In the Clerk’s Office of the District Court of the United States for the 
Southern District of New York. 

Steele’s NAT. PHIL. 

Gift 

Judge and Mrs, Isaac R. Hitt 
July 3, 1933 






TO 


Pm pm*, 

■whose 

SYMPATBCY AN]© ASSBSTAN)©! 
IN MY SCIENTIFIC STUDIES 

HAVE BEEN 

PM IttSpiratitttt anti 

TfoSs ^Q) IIU1CTH) © 

IS 


Jjedualtd 







.PREFACE. 


This work has grown up in the class-room. It 
contains those definitions, illustrations, and applica¬ 
tions which seemed at the time to interest and in¬ 
struct the pupils. Whenever any explanation fixed 
the attention of the learner, it was laid aside for 
future use. Thus, by steady accretions, the process 
has gone on until a hook is the result. 

As Physics is generally the first branch of Natural 
Science pursued in schools, it is important that the 
beginner should not he disgusted with the abstractions 
of the subject. The Author has therefore endeavored 
to use such simple language and practical illustrations 
as will interest the learner, while he is at once led 
out into real life. Prom the multitude of philosophi¬ 
cal principles, those only have been selected which 
are essential to the information of every well-read 
person. The many curious questions which yet rank 
only as “philosophical gossip,” are rarely mentioned. 
Within the brief limits of a small text-book, no sub¬ 
ject can he exhaustively treated. This is, however, 
of less importance now, when every ‘teacher feels that 
he must of necessity be above and beyond any school- 
work in the fulness of his information. 



8 


PREFACE. 


The theories advanced are those generally received 
among scientific men. The object of an elementary 
work is not to advance the peculiar ideas of any one 
person, but simply to give such currently accepted facts 
as are believed by all. This plan affords no scope for 
original thought. The Author has therefore simply 
sought to gather from every attainable source the fresh¬ 
est and most valuable information, and so weave it to¬ 
gether as to please as well as instruct his pupils. The 
time-honored classifications established by the masters, 
and recognized in all scientific works, have been retained. 
It has not seemed wise to reject familiar terms for a 
mere appearance of novelty. Since the problems are 
for the instruction of American youth, the system of 
weights in general use in this country is employed. 

The object of the Author will be fully attained if 
he succeed in leading some young mind to become a 
lover and interpreter of Nature, and thus come at 
last to see that Nature herself is but a “thought of 
God.” 


Peck’s Ganot’s Natural Philosophy, from the French of 
A. Ganot. By Prof. W. G. Peck, LL.D., Columbia College. 
Adapted to follow this work, or take its place in classes requiring 
a fuller course. 

“ Peck’s Ganot ” is noted for its clear method and its magnifi¬ 
cent system of illustration (from the original French plates), 
largely doing away with the necessity for Apparatus. 

The work contains 500 12mo pages, and is of Academic grade. 
Price $1.75, post-paid. A. S. Barnes & Co., Publishers. 



SUGGESTIONS TO TEACHERS. 


ScnoLARS are expected to obtain information from this book, 
without the aid of questions, as they must always do in their 
general reading. When the subject of a paragraph is announced, 
the pupil should be prepared to tell all he knows about it. The 
diagrams and illustrations, as far as possible, should be drawn upon 
the blackboard and explained. Although pupils may, at first, 
manifest an unwillingness to do this, yet in a little time it will be¬ 
come a most interesting feature of the recitation. The “ Practical 
Questions” given at the close of each general subject have been 
found a profitable exercise in awakening inquiry and stimulating 
thought. I They may be used at the pleasure of the instructor. The 
'equations contained in the text are designed to be employed in the 
solution of the problems. The following works will be found use¬ 
ful in furnishing additional illustrations and in elucidating difficult 
subjects, viz.: Desclianel’s Natural Philosophy, Lockyer’s Guil- 
lemin’s Forces of Nature, Stewart’s Elementary Physics, Herschel’s 
Introduction to the Study of Physical Science, Tomlinson’s Intro¬ 
duction to the Study of Natural Philosophy, Knight’s Cyclopaedia— 
Section on Science and Natural History, Pepper’s Play-book of 
Science, Beale’s How to Work with the Microscope, Schellen’s 
Spectrum Analysis, Lockyer’s The Spectroscope, Nugent’s Optics, 
Chevreul on Colors, Thomson & Tait’s Natural Philosophy, Max¬ 
well’s Electricity and Magnetism, Faraday’s Forces of Matter, 
Youman’s Correlation of Physical Forces, Maury’s Physical Geog¬ 
raphy of the Sea, Atkinson’s Ganot’s Physics, Silliman’s Physics, 
Tyndall’s Lectures on Light, Heat and Sound, Tyndall’s Forms of 
Water, Snell’s Olmsted’s Philosophy, Loomis’s Meteorology, Mil¬ 
ler’s Chemical Physics, Cooke’s Religion and Chemistry, Benedicite, 
and the Illustrated Library of Wonders. They may be procured 
of the publishers of this book. The author will be pleased to cor¬ 
respond with teachers with regard to the apparatus necessary foi 
the performance of the experiments named in the work, or with 
reference to any of the “ Practical Questions.” 


TABLE OF CONTENTS. 


I. INTRODUCTION. 

r es 

MATTER ....... . >3 

General Properties.15 

Specific Properties.27 

II. ATTRACTION ... 33 

MOLECULAR FORCES. 35 

Cohesion.36 

Adhesion.40 

GRAVITATION.4S 

Weight. 

Falling Bodies .5c 

Centre of Gravity.54 

The Pendulum.58 

III. MOTION . . .68 

LAWS OF MOTION.70 

Compound Motion.71 

Composition and Resolution of Forces . 72 

Circular Motion.75 

Reflected Motion.79 

IV. MECHANICAL POWERS .. . 83 

THE ELEMENTS OF MACHINERY ... 85 

Lever. 85 

Wheel and Axle.90 

Inclined Plane. -93 

Screw.95 

Wedge.99 

Pulley. 





TABLE OF CONTENTS. 


11 


V. PRESSURE OF LIQUIDS AND GASES ioi 


HYDROSTATICS.103 

Liquids Influenced by Pressure . . .103 

Liquids Influenced by Gravity alone . . 107 

Specific Gravity .iij; 

HYDRAULICS.122 

Water-wheels.125 

Wave-motion.128 

PNEUMATICS.132 

Properties of the Air. 133 

Pressure of the Air, Barometer, Pumps, Si¬ 
phon, &c..136 


VI. ON SOUND ... 149 

ACOUSTICS.151 

Sound Produced by Vibrations . . .151 

Velocity of Sound. 155 

Intensity. Speaking-tubes, Trumpet, &c. . 157 

Refraction.159 

Reflection. Echoes, &c.160 

Noise and Music. 163 

The Siren, Length of Sound-wave, &c. . 164 

Interference of Sound . . . - .168 

Vibration of Cords.169 

Nodes.171 

Wind Instruments. 176 

The Ear .i 7 8 

Singing Flames.182 


VII. ON LIGHT 


185 


OPTICS. 

Laws and Theory of Light 

Reflection. Mirrors, &c. 

Refraction. Prisms, &c. .... 
Composition. The Spectrum, Diffraction, 
Polarization, the Rainbow, &c. . 


187 

188 
190 
197 

204 








12 


TABLE OF CONTENTS. 


Optical Instruments. Telescope, Microscope, 
Opera-glass, Magic Lantern, Stereoscope, 

Camera, &c.215 

The Eye.220 

VIII. ON HEAT . . 225 

NATURE AND THEORY OF HEAT . . .228 

CHANGE OF STATE BY HEAT ... 232 

Expansion . . . . . . . . 233 

Liquefaction.236 

Vaporization.237 

Evaporation.241 

COMMUNICATION OF HEAT . . . .243 

Conduction.243 

Convection. 244 

Radiation.245 

THE STEAM-ENGINE.246 

METEOROLOGY.248 

IX. ON ELECTRICITY . . 259 

MAGNETIC ELECTRICITY.261 

FRICTIONAL ELECTRICITY .... 269 

GALVANIC ELECTRICITY.287 

ELECTRO-MAGNETISM.302 

ANIMAL ELECTRICITY.314 

CONCLUSION.314 


NOTES ON APPARATUS AND EXPERIMENTS .318 

QUESTIONS.323 

INDEX ...» . 339 










NATURAL PHILOSOPHY. 



INTRODUCTION. 


Matter is anything we can perceive with our 
senses. A body is a distinct portion of matter. Ex.: 
A chair, 1 lb. of iron, a piece of silver. A substance 
is any one of the different kinds of matter. Ex.: 
Gold, wood, stone. 

Properties of Matter.— Each substance possesses 
two kinds of properties —general and specific; the 
former belongs to all substances, the latter only to 
particular ones. Ex.: Gold has weight. This prop¬ 
erty is not peculiar to gold, for all substances have 
weight; hence it is a general property. But gold is 
yellow. This is so distinctive that we speak of a 
“ golden colorhence it is a specific property. A 
piece of glass has form. All bodies have form; 
hence it is a general property. But glass is so brittle 
that we say “ brittle as glasshence brittleness is 
a specific property. 



4 


NATURAL PHILOSOPHY. 


Changes of Matter.— Each substance can un¬ 
dergo two kinds of change —physical and chemical . 
The former does not destroy the specific properties 
of the substance; the latter does. Ex.: An eagle 
is beaten into gold-leaf; but this does not alter the 
color or other specific properties of the metal, hence 
it is a physical change. Melt the eagle in a cruci¬ 
ble, and still the same is true. But put it into an 
acid, and soon it is dissolved—the specific proper¬ 
ties are entirely destroyed; hence it is a chemical 
change. Draw a nail into wire, and the specific 
properties of the iron remain untouched. But leave 
it in a basin of water, and the distinguishing proper¬ 
ties of iron disappear—it becomes brittle, red, soft, 
and scaly; hence this is a chemical change. 

These distinctions give rise to two branches of 
Natural Science, Philosophy and Chemistry. The 
former treats of the physical, the latter of the 
chemical changes of matter. As all bodies possess 
both kinds of properties, and are susceptible of 
both kinds of change, these branches are inti¬ 
mately connected. 


!Practicat Questions.—My knife-blade is magnetized, so that it will pick 
up a needle: is that a physical or chemical change ? Is it treated in Phi¬ 
losophy or Chemistry? 

Is the burning of coal a physical or chemical change? The production 
of steam ? The formation of dew ? The falling of a stone ? The growth 
of a tree ? The flying of a kite ? The chopping of wood ? The explosion 
of powder ? The boiling of water ? The melting of iron ? The drying of 
clothes ? The freezing of water ? 


GENERAL PROPERTIES. 


15 


The General Properties of Matter. 

'We cannot imagine a body which does not pos¬ 
sess all the general properties of matter. The most 
important are Magnitude, Impenetrability, Divisi¬ 
bility, Porosity, Inertia, and Indestructibility. 

Magnitude is the property of occupying space. 
Size is the amount of space a body fills. Every 
body has three dimensions—length, breadth, and 
thickness. In order to measure these, some 
standard is required. Anciently, certain, portions 
of the human body were used for this purpose. 
Ex.: The foot; the cubit, or length of the forearm 
from the elbow to the end of the middle finger; the 
finger’s length or breadth ; the hand’s breadth; the 
span, etc. • 

The English system .—The first intimation that is 
given of an attempt to have a standard in England, 
is that of 1120. King Henry ordered that the ell, 
the ancient yard, should be the exact length of his 
arm. Afterward we learn that a standard yard¬ 
stick was kept at the Exchequer in London; but it 
was so inaccurate, that a commissioner, who exam¬ 
ined it in 1742, wrote: “ A kitchen poker filed at 
both ends would make as good a standard. It has 
been broken, and then repaired so clumsily that the 
joint is nearly as loose as a pair of tongs.” In 
1760, Mr. Bird prepared an accurate copy of this 
for the use of the Government. It was not legally 
adopted until 1824, when it was ordered, that if de- 


i6 


NATURAL PHILOSOPHY. 


stroyed it should be restored by a comparison with 
the length of a pendulum vibrating seconds at the 
latitude of London. (See page 59, 4th Law.) 

At the great fire in London, 1834, the Parliament 
House was burned, and with it Bird’s yard-stick. 
Repeated attempts were then made to find the length 
of the lost standard by means of the pendulum. 
This was found utterly impracticable. At last the 
British Government adopted a standard prepared 
from the most reliable copies of Bird’s yard-stick. 
A copy of this was taken by Troughton, a celebrated 
instrument-maker of London, for the use of our 
Coast Survey.* 

The French Standard .—The French adopted as 
the length of the legal foot, that of the royal foot 
of King Louis XIV., as perishable a standard 
as King Henry’s arm. In 1790, however, they 
took to\7oV,tot length of the quarter of a me¬ 

ridian of the earth’s circumference as the basis of 
all measurements. This is equal to 39 + inches; 
and is called a metre. 

A Natural Standard .—Two attempts have thus 
been made to fix upon something in Nature as an 
invariable unit of measure. The French had, how¬ 
ever, scarcely completed their system, when they 
found that a mistake had been made in measuring 

* This yard is about yoW of an inch longer than the British 
standard. According to Act of Congress, sets of weights and 
measures have been distributed to the governors of the several 
states. The yards so furnished are equal to that of the Troughton 
scale. We have no national standard established by law. 




GENERAL PROPERTIES. 


17 


the meridian. The English philosophers discovered 
a similar error in the calculation of the length of the 
pendulum. Both the French and English systems 
are therefore founded upon arbitrary standards. 

Impenetrability is the property of so occupying 
space as to exclude all other bodies. No two 
bodies can occupy the same space at the same 
time. A book lies upon the table before me; no 
power in the world is able to place another in the 
same space with that. I attempt to fill a bottle 
through a closely-fitting funnel; but before the 
liquid can run in, the air must gurgle out or the 
water will trickle down the sides of the bottle. 

Apparent Exceptions .—In common language, we 
speak of one substance penetrating another. Thus, 
a needle penetrates cloth, a nail penetrates wood, 
etc.; but a moment’s examination shows that they 
merely push aside the fibres of the cloth or wood, 
and so press them closer together. Take a tum¬ 
bler brim-full of water, and then cautiously drop 
in shingle-nails. A quarter of a pound can be 
easily added without causing the water to over¬ 
flow. We shall find the explanation of this, in 
the fact that the surface of the water becomes con-' 
vex. 

Divisibility is that property of a body which al¬ 
lows it to be separated into parts. We have never 
seen a particle so small that we could not make 
it smaller. 

Illustrations. — The thread with which certain 
species of spiders weave their web is composed of 


i8 


NATUBAL PHILOSOPHY. 


four smaller threads; each one of these consists of 
one thousand yet smaller, each of which comes 
from a separate tube in the spider’s spinning- 
machine. A German naturalist, after examining 
a web very carefully, decided that it would take 
4,000,000,000 fibres to form a thread as large 
as a hair of his beard; and as each fibre is com¬ 
posed of 4,000 smaller ones, it follows that each of 
the least fibres is only ttt, o o o ,tto V, o o o ',out p&rt the size 
of a human hair. It is said that a half-pound of the 
full-sized thread would girt the globe. It would re¬ 
quire 50,000,000 pounds of wire to erect a telegraph 
around the earth at the equator. A grain of strych¬ 
nine will impart a flavor to 1,750,000 grains of 
water ; hence, each grain of the liquid will contain 
only t.tts'.ttot of a grain of strychnine, and yet that 
amount can be distinctly tasted. A grain of Ma¬ 
genta will color 50,000,000 grains of water. A piece 
of silver containing only one-billiontli of a cubic 
inch— i. e., being .001 of an inch square—when dis¬ 
solved in nitric acid will render milky a solution 
of a hundred cubic inches of common salt. Each 
cubic inch must then contain Tjnr,TFWirFir,innr of a 
cubic inch of silver. The eye can see the color 
distinctly in, perhaps, a hundredth part of a cubic 
inch, in which there would be present only one ten- 
trillionth part of a cubic inch of silver.* 

* Some idea of the vastness expressed by the word trillion 
may be derived from the following curious calculation. If Adam, 
at the instant of his creation, had commenced to count at the 



GENERAL PROPERTIES. 


19 


Press a puff-ball, and eaclx speck of the cloud 
which flies off, under the microscope proves to be 
a beautiful round orange-ball,—the seed of the 
plant. Much of the fine dust that is revealed to us 
in the atmosphere, by a beam of light shining 
through a crevice, consists of the seeds of minute 
plants, which falling on a damp surface grow into 
mildew or mould. Under a microscope, this be¬ 
comes a fairy forest of trees of a new and strange 
growth. 

In all these instances we have mentioned, the 
divisibility is proved by our senses of taste and 
sight. When our eyes fail, the microscope is called 
in to continue the investigation. While thus, prac¬ 
tically, there is no limit to the divisibility of matter, 
philosophers hold that there is, in theory. 

The Atomic Theory supposes that matter is 
composed of inconceivably minute portions called 
atoms, each having a definite shape, weight, color, 
etc., which cannot be changed by any chemical or 
physical force. As has been happily said, “ What 
God made one in the beginning, man cannot put 
asunder.” No one has ever seen one of these ulti¬ 
mate portions of matter, and we have no absolute 
proof that any exist; but the theory is so conve- 


rate of one every second of time, continuing through all the cen¬ 
turies, he would not yet have nearly completed the first quarter 
of a trillion; and even if Eve had come to his relief, and together 
they had counted day and night, they would not see the end of 
their task and enjoy then first leisure for 10,000 years to come. 



20 


NATURAL PHILOSOPHY. 


nient, especially in chemistry, that it is at present 
generally received. 

Animalcule.— The tiny nations of animalcule fur¬ 
nish most striking illustrations of the divisibility of 
matter and the minuteness of atoms. This is a 
world of which our unaided senses furnish us no 
proof. The microscope alone reveals its wonders. 
In the drop of water that clings to the point of a 
cambric needle, the swarming millions of this min¬ 
iature world live, grow, and die. They swim in 
this their ocean, full of life, frisking, preying upon 
each other, waging war, and re-enacting the 
scenes of the great world we see about us. My¬ 
riads <?f them inhabit the pools of water standing 
along the roadside in summer. They go up in 
vapor and fly off in dust, and reappear wherever 
moisture and heat favor the development of life. 
Yet, minute as they are, they have been fossilized 
(turned to stone), and now form masses of chalk. 
Tripoli, or polisliing-slate, is composed of these re¬ 
mains, each skeleton weighing the TTT.inhr.wiF of a 
grain. If we examine whiting under a powerful 
microscope, we shall find that it is composed of tiny 
shells. Now let our imagination conceive the minute 
animals which formerly occupied them. Many of them 
had simple sack-like bodies, but still they had one or 
more stomachs, and possessed the power of digesting 
and assimilating food. This food, coursing in infinitely 
minute channels, must have been composed of solid as well 


GENERAL PROPERTIES. 


2 


as liquid matter; and finally, at the lowest extreme 
of this descending series, we come to the atoms of 
which this matter itself was composed. 

Porosity is the property of having pores. By 
this is meant not only such pores as are familiar to us 
all, and to which we refer when in common language 
we speak of a porous body, as bread, wood, un¬ 
glazed pottery, a sponge, etc., but a finer kind, 
which are as invisible to the eye as the atoms them¬ 
selves. These pores are caused by the fact that the 
molecules* of which a body is composed, are not in 
actual contact, but are separated by extremely 
minute spaces. 

Size of the spaces compared with the size of the atoms. 
—These spaces are so small that they cannot be dis¬ 
cerned with the most powerful microscope, yet it is 
thought that they are very large as compared with 
the size of the atoms themselves. If we imagine a 
being small enough to live on one of the atoms near 
the centre of a stone, as we live on the earth, then 
we are to suppose that he would see the nearest 
atoms at great distances from him, as we see the 
moon and stars, and might perchance have need of 
a fairy telescope to examine them, as we investigate 
the heavenly bodies. 

Illustrations .—1. Having a bowl full of water, it 

* The word molecule means a little mass. A group of atoms 
forms a molecule, and a collection of molecules constitutes a body. 
Thus a molecule of water is composed of two atoms of hydrogen 
and one of oxygen. (See New Chemistry, Rev. Ed., p. 56.) 



22 


NATURAL PHILOSOPHY. 


is easy to add a large quantity of fine salt without 
apparently increasing the bulk of water in the 
least. We must only be careful to drop in the 
salt slowly, giving it time to dissolve and the bub¬ 
bles of air to pass off. When the liquid has taken 
up all the salt, we can add finely powdered sugar, 
and afterward other soluble solids in the same man¬ 
ner. In this case we suppose that the particles of 
sugar are smaller than those of salt, and those in 
turn smaller than those of water. The particles of 
salt fill the spaces between the particles of water, 
and those of sugar occupy the still smaller spaces 
left between the particles of salt. We may better 
understand this if we suppose a bowl filled with 
oranges. It will still hold a quantity of peas, then 
of gravel, then of fine sand, and lastly some water. 

2. At Florence, Italy, in the 17th century, a hol¬ 
low sphere of gold was filled with water and tightly 
closed. Pressure was then applied to the outside, 
and the ball partly flattened. This change of form 
diminished the size, and so the water was forced 
through the metal and formed on the surface like 
drops of dew. This experiment proved that gold 
has pores, and that they are larger than the mole¬ 
cules of water. 

3. In testing large cannon, water is forced into 
the gun by hydrostatic pressure until it oozes 
through the thick metal and covers the outside of 
the gun like froth, then gathers into drops and 
runs down to the ground in streams. 


GENERAL PROPERTIES. 


23 

4. Over the Menai Straits there is a magnificent 
tubular bridge 100 feet above the water. The tubes 
were floated to the spot in vessels, and then raised 
to their position by means of immense Hydraulic 
Presses. The cylinders of these presses (see R, Fig. 
70) were made of iron a foot thick. Yet when in 
full operation it is said the water would form in 
drops on the outside, and the workmen, rubbing it 
off with their fingers, would speak of the machine as 
“ sweating.” They were at last compelled to partly 
stop these pores by mixing oat-meal with the water 
used in the presses. 

5. Stone pillars and arches are frequently com¬ 
pressed by the great weight which rests upon them. 
The columns which support the dome of the Pan¬ 
theon at Paris are said to have been considerably 
shortened in this manner. 

6. Ashes will “ keep fire ” because they are por¬ 
ous, and permit enough air to pass in to main¬ 
tain a slow combustion and so preserve the coals 
alive. 

7. The process of filtering, so much employed by 
druggists and chemists, depends upon this property; 
the liquid slowly passes through the pores of the 
filter, leaving the solid portions behind. Water, in 
Nature, is thus purified by percolating through beds 
of sand and gravel. Cisterns for filtering water have 
a partition in the middle; one side contains charcoal 


24 


NATURAL PHILOSOPHY. 


and sand, the other the rain-water; as the water 
filters through these substances it is cleansed of its 
impurities. Small filters are frequently made on the 

Fig. 1. 


& 



same principle. They consist of a cask nearly filled 
with gravel and charcoal; the water is poured in a 
little reservoir at the top and drawn off at the bot¬ 
tom by a faucet. 

8. Gases are known to be porous from the fact 
that when a jar is filled with one kind of gas, it will 
contain as much of another kind as if the jar were 
empty. The molecules of the second must spread 
themselves between the molecules of the first. This 
illustrates the principle that “ one gas is a vacuum 
for another gas.” 

















I 


GENERAL PROPERTIES. 2 5 

Inertia is the property of passiveness. Matter 
has no power of putting itself in motion when at rest, 
nor of coming to rest when in motion. A body will 
never change its place unless moved, and if once 
started will move forever unless stopped. If we 
leave our room, and on our return find a book miss¬ 
ing, we know some one has taken it,—the book could 
not have gone off at its own suggestion. We gen¬ 
erally think a body is more inclined to rest than to 
motion; and so, while we see how a stone could not 
throw itself, we find it difficult to understand how, 
once thrown, it does not stop itself. We shall see 
hereafter that several forces destroy its motion and 
bring it to rest. 

Illustrations. —1. When we try to start a heavy 
wagon it requires a great effort, because we have to 
overcome its inertia, which tends to keep it at rest. 
When the wagon is in motion it requires as great an 
exertion to stop it, since then we have again to over¬ 
come its inertia, which tends to keep it moving. 

2. Inertia causes the danger in jumping from a 
car when in rapid motion. The body has the speed 
of the train, while the motion of the feet is stopped 
by the contact with the ground. One should jump 
as nearly as he can in the direction in which the 
train is moving, and with his muscles strained, s« 
as to break into a run the instant his feet touch 
the ground. Then with all his strength he can 
gradually overcome the inertia of his body, and 
after a few rods can turn as he pleases. 


26 


NATURAL PHILOSOPHY. 


‘Practical Questions.—1. If one is riding rapidly, in which direction will 
he be thrown when the horse is suddenly stopped ? 2. When standing in a 
boat, why, as it starts, are we thrown backward ? 3. When carrying a cup of 
tea, if we move or stop quickly, why is the liquid liable to spill? 4. Why, 
when closely pursued, can we escape by dodging? 5. Why is a carriage or 
sleigh, when sharply turning a corner, liable to tip over? 6. Why, if you 
place a card on your finger and on top.of it a cent, can you snap the card from 
under the cent without knocking the latter off your finger ? 


Indestructibility is the property which renders 
matter incapable of being destroyed. No particle 
of matter can be annihilated , except by God, its 
creator. We may change its form, but we cannot 
deprive it of existence. Ex. : We cut down a tree, 
saw it into boards, and build a house. The house 
burns, and only little heaps of ashes remain behind. 
Yet in these ashes, and in the smoke of the burning 
building, exist the identical atoms which have passed 
through these various forms unchanged in shape, 
color, or weight.* 

Compressibility is often given as a general prop¬ 
erty of matter. It is, however, a mere result and 
proof of porosity. It is a distinguishing feature of 
gases. They are readily compressed ; solids require 
more force, while, for a long time, this property was 
denied to liquids, and they are even now practically 
incompressible. Weight is also a property of all 

* Sir Walter Raleigh, while smoking in the presence of Queen 
Elizabeth, offered to bet her majesty that he could tell the weight 
of the smoke that curled upward from his pipe. The bet was ac¬ 
cepted. Raleigh quietly finished, and then weighing the ashes, 
subtracted this amount from the weight of the tobacco he had 
placed in the pipe ; he thus found the exact weight of the smoke. 
The queen is said to have paid the wager, having in this way 
learned something of the indestructibility of matter. 




GENERAL PROPERTIES. 


2 7 


bodies with which we are acquainted. It is not, 
however, essential to our idea of matter, since we 
can conceive of a substance without weight. Weight 
is only the result of attraction. Indeed, if there 
were but one body in the universe it would have no 
weight, since it could not be attracted in any direction. 

The Specific Properties of Matter. 

These are properties which are found only in par¬ 
ticular kinds of matter. The most important are 
Ductility, Malleability, Tenacity, Elasticity, Hard¬ 
ness, and Brittleness. They are doubtless caused by 
modifications in the attraction of Cohesion, of which 
we shall soon speak. 

Ductility. —A ductile body is one which can be 
drawn into wire. In the cut is represented a machine 


Fig. 2. 



for making iron wire. B is a steel drawing-plate 
pierced with a series of gradually diminishing holes. 
A rod of iron, A, is hammered at the end so as to 
pass through the largest of these. It is then grasped 
by a pair of pincers, C, and, by turning the crank, D, 
is drawn through the plate, diminished in size and 
proportionately increased in length. The speed 






28 


NATURAL PHILOSOPHY. 


varies from one to six feet per second, according to 
size and quality. The rod is then passed through 
a smaller hole; and the process is continued until the 
required fineness is reached. The holes in the plate 
are kept well lubricated with grease or wax. The 
wire is strongest when drawn cold. After a few 
drawings the iron loses in part its ductility, and is 
then annealed by being heated in an oven and after¬ 
ward cooled slowly. The tenacity of iron is in¬ 
creased by the process of drawing. A bar one inch 
square, which would sustain 30 tons, on being con¬ 
verted into a coarse Avire rope will sustain 40 tons, 
and into fine wire, even 90 tons. 

Gold, silver and platinum are the most ductile 
metals. A silver rod an inch thick, covered with 
gold-leaf, may be drawn to the fineness of a hair and 
yet retain a perfect coating of gold—3 oz. of the 
latter metal making 100 miles of the gilt-thread used 
in embroidery. Platinum w T ire has been drawn so 
fine that, though it is the densest of the metals used 
for this purpose, being nearly three times as heavy as 
iron, a mile’s length weighed only a single grain. 
(See Revised Chem., page 170, for description of the 
process.) Brass wire is made so small, that when 
woven into gauze there are 67,000 meshes in a square 
inch. 

Malleability.— A malleable body is one which 
can be hammered or rolled into sheets. Gold is 
one of the most malleable of metals. Gold-leaf is 
prepared in the following manner. An ingot of 


SPECIFIC PROPERTIES. 


29 


gold is passed many times between steel rollers, 
which are so adjusted as to be brought constantly 
nearer together. An ounce of gold is thus reduced to 
a ribbon one inch wide and 15 feet long. This is 
cut into pieces an inch in length. 150 of these are 
piled up alternately with leaves of strong paper four 
inches square. A workman with a heavy hammer 
beats this pile until the gold is spread to the size of 
the leaves. Each piece is next quartered and the 
600 squares are placed between leaves of gold¬ 
beater’s skin and re-pounded. They are then taken 
out, spread by the breath, re-cut, and the 2,400 
squares re-pounded as before. The beating may be 
continued until 360,000 leaves make only an inch in 
thickness. They are finally trimmed and placed 
between the pages of little books, each of which 
contains 25 gold leaves. 

Copper is so malleable , that it is said that a work¬ 
man, with his hammer, can beat out a kettle from a 
solid block of the metal. 

Tenacity.— A tenacious body is one which cannot 
be easily pulled apart. Iron is the most tenacious 
of the metals. A wire .078 of an inch in diameter, 
will sustain a weight of nearly 450 lbs., while one of 
lead would be broken by a weight of 28 lbs. 

Elasticity is of three kinds: Elasticity of Com¬ 
pression, Elasticity of Expansion, and Elasticity of 
Torsion, according as a body tends to resume its 
original form when compressed, extended, or hoisted. 

Elasticity of Compression .—1. Many solids possess 



NATURAL PHILOSOPHY. 


30 

this property in a high degree. A sword was ex¬ 
hibited at the World’s Fair in London, which could 
be bent into a circle, and 
on being released would 
fly back and become 
straight again. The elas- 
^ ticity of ivory may be 

shown by the following 
experiment. Spread a 
thin coat of oil on a 
smooth marble slab. If 
an ivory ball be dropped 
upon it, the size of the 
impression made will 
vary with the distance at 
which the ball is held 
above the table. This 
shows that the ivory is 
flattened, somewhat as is a soap-bubble when it 
strikes a smooth surface and rebounds. Putty and 
clay are slightly elastic. 2. Liquids are compressed 
with great difficulty ;* but when the force is removed 
they regain their exact volume. They are therefore 
perfectly elastic. 3. Gases are easily compressed, 



Fig. 3. 



* Tims, for a pressure of one atmosphere, 15 lbs. per square inch, 
the diminution of volume of the following liquids is only, as com¬ 


pared with the original volume- 

1. Water, t,W o,Voir 

2. Mercury, 

3. Ether, r.wVtwr 


4. Alcohol, t,w~§\Vw 

5. Chloroform, t,Wo\W“o 

6. Sea-water, t,wo\Vw 






SPECIFIC PROPERTIES. 


31 


but are also perfectly elastic. A pressure of 15 lbs. 
to the square inch reduces the bulk of water only 
ts-.Wo part, whereas it diminishes the volume of a 
gas one-half. A gas may be kept compressed for 
years, but will instantly return to its original form 
on being released. 

Elasticity of Expansion .—This property is possessed 
largely by solids, slightly by liquids, and not at all 
by gases. Ex.: India-rubber, when stretched, tends 
to fly back to its original dimensions. When a solid 
remains stretched for any length of time it loses its 
elasticity. For this reason a violin is unstrung when 
not in use. A drop of water hanging to the nozzle 
of a bottle may be touched by a piece of glass and 
drawn out to considerable length. When let go, it 
will resume its spherical form. Gases manifest no 
tendency to return to their original dimensions when 
extended. 

Elasticity of Torsion is the ten¬ 
dency of a thread or wire which has 
been twisted, to untwist again. It 
is a most delicate test of the 
strength of a force, and is of great 
service in accurate measurements in 
physical science. 

Hardness. —A hard body is one 
which does not readily yield to 
pressure. One body is harder than ^<gjj | 
another when it will scratch or in¬ 
dent it. This property does not de- A Torsion Balance. 

2 * 


Fig. 4. 








32 


NATURAL PHILOSOPHY. 


pend on density.* Ex. : Gold is denser than iron, 
yet is much softer. Mercury is a liquid, yet it is 
twice as dense as steel. 

Brittleness.— A brittle body is one that is easily 
broken. It is a frequOnt characteristic of hard 
bodies. Ex. : Glass will scratch iron, and is ex¬ 
tremely brittle. 


* Density indicates the quantity of matter contained in a given 
bulk. A dense body has its molecules very closely compacted. 
The word rare , which is the opposite of dense, is generally ap¬ 
plied to gases. 




ttraqtion 


♦ 


“ The smallest dust which floats upon the wind 
Bears this strong impress of the Eternal mind: 
In mystery round it subtle forces roll, 

And gravitation binds and guides the whole.” 









w 








MOLECULAR FORCES. 

Molecular forces exist in tlie molecules of mat¬ 
ter, and act only at insensible distances. 

If we take a piece of iron and attempt to pull it 
to pieces, we find that there is a force which holds 
the particles together and resists our efforts. If we 
try to compress the metal, we find that though there 
are pores in it and the molecules do not touch each 
other, yet there is a force which holds the particles 
apart and resists our efforts as before. If we apply 
heat, the iron expands and finally melts. If, in 
like manner, we heat a bit of ice, we notice that the 
attractive force is gradually overcome, the solid be¬ 
comes a liquid, and finally the repulsive force pre¬ 
dominates and the liquid passes off in vapor. In 
turn, we can cool the vapor, and convert it back 
again into water and ice. We thus see that there 
are two opposing forces wdiich reside in molecules— 
an Attractive and a Repulsive force, and that the 
latter is Heat. There are three kinds of the former, 
Cohesion, Adhesion , and Chemical Affinity 

* Chemical affinity produces chemical changes, and its consid¬ 
eration belongs entirely to chemistry. It binds together atoms or 
molecules of different kinds, and causes them to form new com¬ 
pounds. 



36 


NATURAL PHILOSOPHY. 


Cohesion is the force which holds together mole¬ 
cules of the same kind. 

The three states of Matter. —Matter is found in 
three states, solid, liquid, and gaseous. These de¬ 
pend on the relation of the Attractive and Repulsive 
forces—Cohesion and Heat. If the attractive force 
is the stronger, the body is solid ; if they are nearly 
balanced, it is liquid: if the repulsive force is 
stronger, it is gaseous. Most bodies may be made to 
take these three states successively. Thus, by the 
addition of heat, ice may be converted into water 
and thence into vapor, or vice versa , by the subtrac¬ 
tion of heat. Most solids pass easily to the liquid 
form, others go directly from the solid to the gas¬ 
eous state. 

Cohesion acts at insensible distances. —Take two bul¬ 
lets, and having flattened and cleaned one side of 
each, press them together with a slightly twisting 
motion. They will be found to cohere when the 
molecules are crowded into apparent contact. Two 
panes of plate-glass, which accidentally fall against 
each other, are thus brought within the range of the 
attraction of Cohesion, and are frequently cut and 
polished as one pane. If two globules of mercury 
be brought near each other, they remain separate 
until the instant they seem to touch, when they im¬ 
mediately coalesce. Two freshly-cut surfaces of 
rubber, when slightly warmed and pressed together, 
cohere as if they formed but one piece. 

Welding .—This process illustrates the principle just 


COHESION. 


37 


named. A rod of iron being broken, we wish to mend 
it. So we bring the iron to a white-heat at the ends 
which we intend to unite. This partly overcomes 
the attraction of Cohesion, and the molecules will 
move easily upon one another. Laying now the two 
heated ends upon each other, we pound them with a 
heavy hammer until over both surfaces the molecules 
are brought near enough for the attraction of Co¬ 
hesion to bind them together. Iron and platinum 
are the only metals which can be welded, as they 
are the only ones which become softened just before 
melting. The same property is possessed in a re¬ 
markable degree by glass. Gutta-percha, when 
warmed in water, can also be welded. Dough, wax, 
and butter can be readily united at common tem¬ 
peratures. 

Liquids tend to collect in spheres .—Mix water and 
alcohol in such proportions that a drop of sweet-oil 
will fall just to the centrfe of the fluid. In this way 
the attraction of the earth is neutralized, and the 
molecules of the oil are left free to arrange them¬ 
selves as they please. The drop will form a perfect 
sphere. The same tendency is seen in dew-drops, 
rain-drops, globules of quicksilver, in the manufac¬ 
ture of shot, etc.* The reason for this is simply 
that the force of Cohesion acts toward the centre of 
the drop. In the spherical body, every portion of 
the surface is equally distant from the centre; and 
when that form is assumed every molecule on the 


* See Fourteen Weeks in Chemistry, Rev. Ed., p. 174. 



NA TUBAL PHILOSOPHY. 


3S 

outside is equally attracted, and there is an equilil> 
rium established. 

Solids tend to form regular crystals .—When a liquid 
becomes a solid, the general tendency is to assume 
a symmetrical form. The attraction of Cohesion 
strives to arrange the molecules in an orderly man¬ 
ner. Each kind of matter has its peculiar shape 
and angle, by which its crystals may be recognized. 
Even when different substances are contained in the 
same solution, they separate on crystallization. The 
beautiful finish and perfection of the crystals thus 
formed in nature infinitely transcend the workman¬ 
ship (rf the highest art. God delights in order as in 
beauty. Down in the dark recesses of the earth He 
has fashioned, by the slow processes of His laws, 
the rarest gems—amethysts, rubies, and diamonds. 
There are mountain masses transparent as glass, 
caves hung with stalactites, crevices rich with gold 
and silver, and lined with -quartz. Everywhere we 
find regularity and symmetry. This tendency is seen 
in the beautiful crystalline forms of snow-flakes and 
of frost. A mass of ice seems irregular, yet if closely 
examined it reveals the perfect crystals crowded to¬ 
gether by the rapidity with which the solidification 
took place. If we watch the surface of water which 
is slowly freezing, we can see the regular arrange- 
ment of the long crystals as they shoot out from 
each side of the vessel. The very soil is largely 
composed of broken and decomposed crystals worn 
down from the rocks by the action of the rain and frost. 



COHESION. 


39 

Illustrations .—TVe can illustrate the formation of 
crystals by adding alum to hot water until no more 
will dissolve; then, suspending strings across the 
dish, setting it away to cool. Beautiful octahedral 
crystals will collect over the threads and the sides 
of the vessel. The slower the process the larger 
will be the crystals. To form the massive crystals 
found in nature has doubtless required centuries. 
The large ones seen in the show-windows are made 
by “feeding” a single small perfect crystal every 
day with a fresh solution. Melted iron rapidly 
cooled in a mould has no time to arrange its 
crystals perfectly. If, however, the iron be after¬ 
ward jarred, as when used for heavy cannon, the 
axles of rail-cars, etc., the molecules take on the 
crystalline form and the metal becomes brittle. 
On examining such a piece of iron we can see in a 
fresh fracture the smooth, shiny face of the crystals. 

Tempering and annealing illustrate a curious prop¬ 
erty of cohesion. A piece of iron is heated and then 
plunged into oil or water. It becomes hard and 
brittle. If, on the contrary, it be heated and cooled 
slowly, it is made tough and flexible. Strangely 
enough, the same process which hardens iron softens 
copper. It is supposed that the arrangement of the 
jnolecules, and the consequent strength of the metal, 
depend on the time occupied in cooling. Steel is 
tempered by heating white-hot, then cooling quickly, 
and afterward re-heating and cooling slowly. The 
more it is re-heated the softer it becomes. 


40 


NATURAL PHILOSOPHY . 


A Prince Rupert's Drop , or Dutch tear, consists 
simply of a tear of melted glass dropped into water, 
and so cooled quickly. The outside forms in regu¬ 
lar crystals, while the inner portion, not having room 
to expand, causes a violent strain upon the exterior. 
The outer shell is strong enough to resist quite 
a heavy blow with a hammer, but if the small end 
be nipped off, the whole mass flies into powder with 
a sharp explosion. 

Practical Questions .— 1 . Why can we not weld a piece ol copper to one 
of iron? 2. Why is a bar of iron stronger than one of wood ? 3. Why is a 
piece of iron, when perfectly welded, stronger than before it was broken ? 4. 
Why do drops of different liquids vary in size ? 5. When you drop medicine, 
why will the last few drops contained in the bottle be of a larger size than the 
others ? 6. Why are the drops larger if you drop them slowly ? 7. Why is a 
tube stronger than a red of the same weight ? 8. Why, if you melt scraps of 
lead, will they form a solid mass when cooled ? 9. In what liquids is the force 
of cohesion greatest? 10. Name some solids which will volatilize without 
melting. SA? 

Adhesion is tlie force which holds together mole¬ 
cules of different kinds. Ex.: We fasten together 
two pieces of wood with glue, two pieces of china 
with cement, two bricks with mortar, two sheets of 
paper with mucilage, two pieces of tin with solder, 
glass and wood with putty, glass and brass with 
plaster of Paris, and paper to the wall with paste. 
Paint adheres to the wood-work, dust to the wall, 
and chalk to the blackboard. 

The adhesion between animal charcoal and va¬ 
rious coloring matters is very great. If any liquid 
containing these substances be filtered through 
it, the foreign matter in solution will adhere to 
the charcoal, while the liquid will run through 


ADHESION. 


41 


Fi*. 5. 


perfectly colorless. Syrup is thus purified by 
passing through a layer of charcoal 12 or 13 
feet thick. The cleansing qualities of common 
charcoal in water-filters is probably due largely to 
this property. Bubbles can be 
blown from soapsuds, because 
the soap by its adhesive force 
holds the particles of water to¬ 
gether. : . . 

Capillary Attraction (capillus, 
a hair) is a variety of adhesion. 

It may be seen when two 
plates of glass are placed in 
water, as shown in Fig. 5, but 
is exhibited most strikingly in very Fi s. 6. 
fine tubes, whence the name.f 
1. If we insert a small glass tube 
in water, the liquid will rise in 
the tube. The smaller the tube, the 
greater will be the height. In this 
case, it is evident that the adhe¬ 
sive attraction of the glass is greater 
than the cohesive attraction of the' 
water. There is an attraction between the glass 
and water. 




* If a bubble be blown at the end of a glass tube, the thin film 
of waler contracting by its cohesive force will frequently drive 
back the air through the pipe with sufficient strength to extin¬ 
guish the flame of a candle. 

f These tubes may be easily drawn to any length and size, from 
French-glass tubing, in the heat of a common alcohol-lamp. 










42 


NATURAL PHILOSOPHY. 


2. If we insert a glass tube in a dish of mercury, 
the capillary action is reversed and the height of the 
Fig. 7. liquid is less than the general level. 

In this case the adhesive attraction of 
the glass is less than the cohesive at¬ 
traction of the mercury. There is an 
apparent repulsion between the glass 
and mercury. 

Illustrations. —1. The wick of an oil- 
lamp or a candle is a bundle of fine 
capillary tubes or pores which elevate 
the oil or melted fat and feed the flame. Thus ex¬ 
tinguishers are needed to an alcohol-lamp, because 
by capillary attraction the liquid tends to rise to the 
top and there evaporate until the lamp is emptied. 

2. If the end of a towel be dipped in a basin of 
water, the whole towel will soon be wet by capillary 
action through the fine pores and tubes of the cloth. 
Thus also the capillary tubes of a towel dry one’s 
face after washing. 

3. Blotting-paper absorbs ink by means of its 
capillary tubes. 

4. Water poured in the saucer of a flower-pot is 
elevated through the pores of the earth to the 
plant. 

5. By means of the capillary force water is drawn 
up through the earth to the surface of the ground, and 
there moistens the roots of plants and supplies them 
with the materials of growth. In the winter, when the 
surface is frozen, the water still finds its way upward, 






ADHESION. 


43 


freezing into ice, which on melting in the spring 
produces mud, even where there has been but little 
rain or snow. Ploughing ground causes it to endure 
drought better, because it stirs the soil and increases 
the size of the capillary pores, thus partially pre¬ 
venting the water from being carried to the surface 
and there evaporated. 

6. Eopes absorb water by capillary action, swell, 
and are shortened. Clothes-lines are thus tight¬ 
ened and sometimes broken in a shower. A rope 
will shrink with such force as to lift a great weight.* 

7. Houses are rendered damp by moisture drawn 
in by the capillary action of the pores in -the wood 
or stone walls. 

8. Millstones in Germany are split off by means 
of wooden wedges. These being driven in when 
dry, afterward absorb moisture, swell, and burst the 

* A curious illustration of this is given in the following story. 
When the great Egyptian obelisk was to be raised in the square 
of St. Peter’s, at Rome, Pope Sixtus V. proclaimed that no one 
should utter a word aloud until the engineer announced that all 
danger was passed. As the majestic column ascends, all eyes 
watch it with wonder and awe. Slowly it rises, inch by inch, 
foot by foot, until the task is almost completed, when the strain 
becomes too great. The huge ropes yield and slip. The workmen 
are dismayed and fly wildly to escape the impending mass of stone. 
Suddenly a voice breaks the silence. “ Wet tlie ropes rings out 
clesa.’-toned as a trumpet. The crowd look. There, on a high 
post, standing on tiptoe, his eyes glittering with the intensity of 
excitement, is the architect Zapaglia. Ilis voice and appearance 
startle every one, but his words inspire. He is obeyed. The 
ropes swell and bite into the stone. The column ascends again, 
and in a moment more stands securely on its pedestal. 



44 


NATURAL PHILOSOPHY. 


rock, thus saving an immense expenditure of time 
and money. 

Solution .—If we put a little sugar in water, it 
will dissolve because the adhesive force of the water 
is stronger than the cohesive force of the sugar. As 
heat weakens the cohesive force, it commonly 
hastens solution; and we can dissolve more of a 
substance, and more rapidly, in hot water than in 
cold. In like manner pulverizing a solid hastens 
its solution. A solid will not dissolve in a liquid 
if there is no adhesion between them. Water 
absorbs great quantities of the various gases by 
means of adhesion. It always contains air, which 
renders it pleasant to the taste. In simply pour¬ 
ing it from one dish to another, we notice that 
bubbles of air adhering to the stream are carried 
down, and then rise to the surface and break. It 
has been proposed to apply this principle to the 
ventilation of mines. As both pressure and cold 
weaken the repulsive force of the gases, they favor 
the adhesion between the molecules of the gases 
and those of water. Soda-water receives its effer¬ 
vescence and pungent taste from carbonic acid 
gas, which, being absorbed under great pressure, 
escapes in little sparkling bubbles as soon as the 
pressure is removed. 

Diffusion of liquids .—Let a tall jar be partly filled 
with water colored by blue litmus. Then, by means 
of a long funnel-tube, pour a little clear water con¬ 
taining a few drops of oil of vitriol to the bottom, be- 


/ 


ADHESION. 


4 5 



neath the colored water. The two Fi ?- s. 
will be distinctly defined at first, 
but in a few days they will mix 
throughout, as will be seen by the 
change of color from blue to red. 

A drop of oil of vitriol may thus 
be distributed through a quart of 
water. Most liquids will mingle 
when brought in contact. If, how¬ 
ever, there be no adhesion between 
their molecules, they will not mix, 
and will even separate when thor¬ 
oughly shaken together. 

Diffusion of gases .—Hydrogen gas is only pi 9 
^ as heavy as common air. Yet, if two 
bottles be arranged as in the figure, the 
lower one filled with the heavy gas and 
the upper with the lighter, the gases will 
soon be found thoroughly mingled. 

Osmose of liquids .—When liquids are sepa¬ 
rated from each other by a thin porous 
substance, they do not mingle uniformly, 
but the interchange is modified in a most 
curious manner, according to the nature of 
the liquid and the substance used. At the 
end of a glass tube, as in Fig. 10, fasten a bladder 
full of alcohol. Fill the jar with water, and mark 
the height to which the alcohol ascends in the tube. 
The column will soon be found to be gradually 
rising. On examination we shall see that the alco- 












46 


NATURAL PHILOSOPHY. 


Fi s- 10 - hol has been passing out 

through the pores of the 
bladder and mixing with 
the water, while the water 
has been coming in more 
rapidly. This has been 
explained by supposing 
that the water adheres 
more strongly than the 
alcohol to the bladder. 
Thus, by capillary attrac¬ 
tion it is drawn through 
the membrane, and on 
the inner side, by the 
law of diffusion of liquids, 
mingles with the alcohol. 
By a similar process some 
alcohol passes outward and mixes with the water. 
Whatever liquids are used, that one which wets 
the membrane most readily will pass through 
most rapidly. If we should use a collodion bal¬ 
loon, instead of a bladder, the effect would be 
reversed. 

Osmose of gases .—The following experiment would 
seem to render it probable that there is a similar 
osmose of gases. Fit a small porous cup, such 
as is used with Grove’s Battery, with a cork and 
glass tube, as shown in Fig. 11. Fasten the tube 
so that it will just dip below the surface of the 
water in the lower jar. Now invert over the porous 














ADHESION. 


cup a receiver of hydrogen 
gas. This gas will pass 
through the pores of the cup 
and down the tube so rapidly 
as to bubble up through the 
water almost instantly. 

Rose balloons, so popular 
as toys, soon lose their 
buoyancy, because the hy¬ 
drogen escapes through the 
pores of the rubber much 
more rapidly than the air 
comes in to take its place. 

The balloon soon shrinks 
and drops down from the 
weight of the rubber. 

radical Questions .—1. Why does cloth shrink when wet ? 2. Why 
do sailors at a boat-race wet the sails? 3. Why does not writing-paper blot ? 
4 . Why does paint prevent wood from shrinking? 5. What is the shape of 
the surface of a glass of water ? One of mercury? 6. Why can we not dry 
a towel perfectly by wringing? 7 . Why will not water run through a fine 
sieve when the wires have been greased? 8. Why will gamphor dissolve in 
alcohol and not in water? 9. Why will mercury rise in zinc tubes as water 
will in glass tubes? 10. Why is it so difficult to lift a board out of water? 
11. Why will ink spilled on the edge of a book extend farther inside than if 
spilled on the side of the leaves ? 12. If yon should happen to spill some ink 

on the edge of your book, ought you to press the leaves together? 13. Why 
can you not mix water and oil? 14. What is the object of the spout on a 
pitcher ? Ans.— The water would run down the side of the pitcher by the 
force of Adhesion, but the spout throws it into the hands of Gravitation before 
Adhesion can catch it. 13. Why will water wet your hand, while mercury 
will not? 16. Why is a pail or tub liable to fall to pieces if not filled with 
water or kept in the cellar? 1 7 . Name instances where the attraction of 
adhesion is stronger than that of cohesion. 


4 7 

Fig. 11. 























ATTRACTION OF GRAVITATION. 


We have spoken of the attraction existing be- 
tween the molecules of bodies at minute distances. 
We now notice another form of the same attrac¬ 
tion, which acts between masses at all distances. 

Grand law of Gravitation.*— Every particle of 
matter in the universe attracts every other particle 
of matter with a force directly proportional to its 
mass, and decreasing as the square of the distance 
Fig. 12 . increases. 

Illustrations .—A stone falls to the 
ground because the earth attracts 
it; but in turn the stone attracts 
the earth. The force of the attrac¬ 
tion is in proportion to their rel¬ 
ative mass. They each move to 
meet the other, but the stone 
passes through as much greater 
distance than the earth 'as its 
mass is less. A plumb-line hang¬ 
ing near a mountain is attracted 
out of a perpendicular. In the 
figure, A B represents the ordi¬ 
nary position of the line, while A C indicates the 
attractive power (exaggerated) of the mountain. 




this 


See “Fourteen Weeks in Astronomy,” p. 34, for history of 
law. 





GRA VITA TIOAJ 


49 


Tliis law is not confined to our own world. By 
it the heavenly bodies are bound to each other, and 
thus kept in their orbits. It may help us to con¬ 
ceive how the earth is supported, if we imagine the 
sun letting down a huge cable, and every star in 
the heavens a tiny thread, to hold our globe in its 
place, while it in turn sends back a cord to every one. 
So we are bound to them and they to us. Thus 
the worlds throughout space are linked together by 
these cords of mutual attraction, which, interweaving 
in every direction, make the universe a unit. 

Gravitation is the general term applied to the 
attraction that exists between all bodies in the 
universe. Gravity is used to designate the earth’s 
attraction for all terrestrial bodies; it tends to draw 
them toward the centre of the earth. Weight is 
the measure of the force of gravity. When we 
say that a body weighs 10 lbs., we mean that the 
earth attracts it that amount. The following gen¬ 
eral principles will explain the various phenomena 
of weight. 

I. The iveight of a body at the centre of the earth 
is nothing, because the attraction is there equal in 
every direction. 

II. The weight of a body above the surface of the 
earth decreases as the square of its distance from 
the centre of the earth increases. Ex.: A body at 
the surface of the earth (4000 miles from the centre) 
weighs 100 lbs. What would be its weight 1000 
miles above the surface (5000 miles from the centre) 


5° 


NATURAL PHILOSOPHY. 


of the earth? Solution —(5,000 m.) 2 : (4,000 m.) c :: 
100 lbs. : ;c=64 lbs. 

III. The iveight of a body varies on different 'por¬ 
tions of the surface of the earth.* It will be least at 
the equator, (1) because, on account of the bulging 
form of our globe, a body is there pushed out 
from the mass of the earth, and so removed from 
the centre of attraction ; (2) because the centrifugal 
force is there the strongest. It will be greatest at 
the poles, (1) because, on account of the flattening 
of the earth, a body is there brought nearer its 
mass and the centre of attraction; (2) because 
there is no centrifugal force at those points. 

Falling Bodies.— Since the attraction of the 
earth is toward its centre, all bodies falling freely 
move in a direct line toward that point. This 
line is called a vertical or plumb line (Plumbum, 
lead, because a lead weight is generally used by 
mechanics). All plumb-lines point toward the cen¬ 
tre of the earth. 

Laivs of Falling Bodies. —I. Under the influence of 
gravity alone , all bodies fall with equal rapidity. 

This is well illustrated by the “ Guinea and 
feather experiment.” Let a coin and a feather be 
placed in a long tube, as in Fig. 13, and the air 
exhausted. Quickly invert the tube, and the two 


*In these statements concerning weight, a spring-scale is sup¬ 
posed to he employed. With a pair of balances, the weights used 
would become heavier or lighter in the same proportion as the 
body to be weighed. 




GRAVITATION. 


51 


bodies will fall in the same time. Let in the air 
again, and now th6 feather Fig.it, 

will come fluttering down long 
after the coin has reached the 
bottom. Hence we conclude 
that in a vacuum all bodies 
descend with equal velocity, 
and that the resistance of 
the air is the cause of the 
variation we see in the falling 
of light and heavy bodies. 

The same fact may be noticed 
in the case of a sheet of paper. 

When spread out, it merely 
flutters to the ground; but 
when rolled together in a com¬ 
pact mass, it falls like lead. 

In this case we have not in¬ 
creased the force of attraction, 
but we have decreased the 
resistance of the air. 

II. In the first second , a body gains a velocity of 32 
feet and falls 16 feet .—This has been proved by ex¬ 
periments with the pendulum, and with Atwood’s 
machine, an instrument constructed with great ac¬ 
curacy for such investigations. It will be noticed 
that 16 feet, the distance passed through the first 
second, is the mean between 0, the velocity at the 
beginning, and 32, the velocity at the close of the 
second. 









52 


NATURAL PHILOSOPHY. 


III. In any succeeding second , the velocity is 16 feet 
multiplied hy the corresponding even number 4, 6, 8, 
etc.; and the distance is 16 feet multiplied hy the corre¬ 
sponding odd number ,* 3, 5, 7, 9, etc. 

1. The body commences the second second with 
a velocity of 32 feet, and as gravity is a constant 
force, gains 32 feet more during the second, making 
64 feet —4x16 feet. It commences the third sec¬ 
ond with a velocity of 64 feet, and gains 32 feet 
more; making 96 feet=6x16 feet. 2. The mean 
between 32 feet, the velocity at the beginning of 
the second second and 64 feet, the velocity at the 
close, is 48 feet=3xl6 feet. The mean between 
64 feet, the velocity at the beginning of the third 
second and 96 feet, the velocity at the close, is 80 
feet = 5x16 feet. Hence we conclude that the 
velocities are as the even numbers, and the dis¬ 
tances as the odd numbers. 

IY. In any number of seconds , a body falls 16 feet 
multiplied by the square of the number of seconds. 

We have just seen that a body falls 16 feet the 
first second, and 48 feet the second. Hence in two 
seconds it falls 16 feet+48 feet=64 feet=2 2 xl6 
feet. In three seconds it falls 16+48 + 80 feet= 
144 feet=3 2 xl6 feet. 

Equations of falling bodies. —If we represent the 

* The odd number corresponding to any second is easily found 
by doubling the number of the second and subtracting 1 from 
the result. Ex.: Required the odd number for the eighth second. 
SX2=16. 16—1=15, the eighth odd number. 



GRAVITATION . 


53 


velocity of a falling body by v , the distance by d, 
and the time by t, the following equations can be 
derived from the foregoing laws. 

v = 321 .... (l). 

d = lti 2 .... (2). 

V 2 = 64 d .... ( 8 ). 

If, now, we let g represent the constant force of 
gravity, 16 feet in each second, we have from the (3) 

v = 2Vgd . . . . ( 4 ). 

Easy way of finding the depth of a tvell .—Let a 
stone fall into it, and, with a watch or by the beat 
of the pulse, count the seconds that elapse before 
you hear it strike the bottom. Square the number 
of seconds, and multiply 16 feet by the result. The 
product is the depth. A little time is required for 
the sound to come to the ear, but this is so slight 
that it may be neglected. 

When a body is thrown upward the same principles 
apply, only in a reverse manner. Through the in¬ 
fluence of gravity it loses 32 feet in velocity each 
second it rises. The velocity necessary in order to 
elevate it to a certain point, must be that which it 
would acquire in falling that distance. It will rise 
just as high in a given time as it would fall in the 
same time. If a ball be thrown vertically into the 
air, it will be as long in falling as in rising. In 
theory, it will strike the earth with the same force 
with which it was thrown : in practice, however, the 


54 


NATURAL PHILOSOPHY. 


ball loses about of its force in rising and an equal 
amount in falling, owing to the resistance of the air. 
~t£5entre of Gravity.— This is that point on which, 
\£f supported, a body will balance itself. The line 
of!“direction is a vertical line drawn from the centre 
of gravity. It is the line along which the centre of 
gravity would pass, if the body should fall. When 


Fig. 14. 




a body is at rest, the force of gravity 
which attracts it is said to be in 
equilibrium. 

The three "states of equilibrium are 
stable , unstable , and indifferent, '‘jfijp 

I. A body is in stable equilibrium 
when the centre of gravity is below 
the point of support, or when any 
movement tends to raise the centre 
of gravity. In Fig. 14 a man has 
the centre of gravity lowered below 
the point of support by means of lead 
balls. Remove these 
and he immediately 
falls, but with them he 
is in stable equilib¬ 
rium. Any movement 
tends to raise the cen¬ 
tre of gravity, and he 
returns quickly to a 
state of rest. The toy 
in Fig. 15 illustrates a 
paradox in philosophy, 






GRAVITATION. 


55 



viz.: “When a body tends to fall, liang a weight 
on the heavy side to steady it.” A Fig. 16 . 
needle may be easily balanced on 
its point by means of a cork and 
two jack-knives (Fig. 16), which 
lower its centre of gravity. 

II. A body is said to be in un¬ 

stable equilibrium, when the centre 
of gravity is above the point of support, or when 
any movement tends to lower the centre of gravity. 
If we take the cork, as balanced in Fig. 16, and 
invert it, 'we shall find it very difficult to balance 
the needle ; and, if we succeed, it will readily topple 
off, because the least motion tends to lower the 
centre of gravity. Fig. 17 . 

III. A body is said to be in in¬ 
different equilibrium when the cen¬ 
tre of gravity is at the point of 
support, or when any move¬ 
ment tends neither to ele¬ 
vate nor lower the centre of 
gravity. A ball of uniform 
density on a level surface 
will come to rest in any 
position, because the cen¬ 
tre of gravity moves in a 
line parallel to the floor. 

The centre of gravity may 
l>e found either by balancing 
the body or by suspending it from one corner, as in 




NA TURAL PHIL OSOPH Y. 


56 

Fig. 17. By means of a plumb-line, obtain the 
line of direction, A E; then hang it the same way 
from another corner, and mark the line of direction, 
B D. The point C, where the two lines cross, is 
the centre of gravity. 

The following general principles will be readily 
apparent. 

a. The centre of gravity in a body always tends 
to seek the lowest point. 

b. A body will never tip over while the line of 
direction falls within the base, but will do so as 
soon as it falls without. 

c. The higher the centre of gravity must be 
raised before the line of direction will fall outside 
of the base, the firmer a body stands. 

d. The lower the centre of gravity lies in a body, 
the more stable it is. 

e. In general, narrowness of base combined with 
height tends to instability; while breadth of base 
and lowness produce stability. The celebrated 
leaning tower of Pisa, in Italy, illustrates the prin¬ 
ciples of gravity. It is about 180 feet high, and its 
top leans 15 feet, yet the line of direction falls so 
far within the base that it is perfectly stable, as it 
has stood for seven centuries. The feeling experi¬ 
enced by a person who for the first time looks down 
from the lower side of this apparently impending 
structure, is startling indeed. The towers of Bologna 
(Fig. 18) are also very wonderful. The lower of these 
is 130 feet high and is inclined eight feet from the 
perpendicular. 


GRAVITATION. 


57 


Fig. 18. 



Leaning Tower at Bologna. 


Physiological facts .—Our feet and the space be¬ 
tween them form the base on which we stand. By 
turning our toes outward we increase its breadth. 
When we stand on one foot, we bend over so as to 
bring the line of direction within this narrow base. 
When we carry a pail of water, we balance it by 
leaning in the opposite direction. When we walk 
up hill we lean forward, and in going down hill we 
incline backward, in unconscious obedience to the 
3 * 












wru 






58 


NATURAL PHILOSOPHY. 


Fij?. 19. 


laws of gravity. We bend forward when we wish to 
rise from a chair, in order to bring the centre of 
gravity over our feet; our muscles not having suf¬ 
ficient strength to raise our bodies without this aid. 
When we walk, we lean forward, so as to bring the 
centre of gravity as far in front as possible. Tlius, 
walking is a process of falling. When we run, we 
lean farther forward, and so fall faster. (Phys., p. 49.) 

The pendulum consists of a weight so suspended 
as to swing freely. Its movements to and fro are 
termed vibrations or oscillations. The path through 
which it passes is called the arc. The extent to 
which it goes in either di¬ 
rection is styled its ampli¬ 
tude. Vibrations performed#, 
in equal times are termed 
isochronous (isos, equal, ai 
clironos , time). 

1st Law .—In the sai 
pendulum, all vibrations of 
small amplitude are iso¬ 
chronous. If we let one of 
the balls represented in 
Pig. 19 swing through a 
short arc, and count the 
number of oscillations per 
minute, we shall find them 
uniform. This property 
of the pendulum was dis¬ 
covered by Galileo when a 
















GRAVITATION. 


59 


boy, while sitting in the cathedral at Pisa and watch¬ 
ing the vibrations of a bronze chandelier which hung 
from the ceiling. Others had seen this before him. 
He first noticed that the swinging lamps measured 
time as well as shed light. 

2 d Law .—The time of vibration is not affected 
by the material of which the weight is composed. 
In Fig. 19, let D be a ball of iron, and C one of wood. 
They will be found to oscillate together. 

3 d Laic .—The times of the vibrations of different 
pendulums are proportional to the square roots of 
their respective lengths. Let A be | the length of 
C, and it will vibrate three times as fast. If B be 
l the length of C, it will vibrate twice as fast. 
Conversely, the lengths of different pendulums are 
proportional to the squares of their 
times of vibration. A pendulum 
which vibrates seconds must be four, 
times as long as one which vibrates 
half-seconds, and sixteen times as 
long as one which vibrates quarter- 
seconds. The apparatus represented 
in Fig. 20, can be used to illustrate 
very clearly the preceding laws. 

4 th Laic .—The time of the vibra¬ 
tion of the same pendulum will vary 
at different places on the earth. It 
will decrease as the square root of 
the force of gravity increases. At \ 
the equator a pendulum vibrates 


Fig. 20. 

Mil 















6o 


NATURAL PHILOSOPHY 


Fig. 21, 


most slowly, and at the poles most rapidly. The 
length of a second-pendulum at New York is 39^ 
inches. 

Centre of oscillation .—The length of a pendulum is 
not its absolute length as measured from one ex¬ 
tremity to the other, but the distance from the 
point of support to the centre of oscillation. The 
upper part tends to move faster than the lower part, 
and so hastens the speed of the pendulum. The 
lower part tends to move slower than the upper part, 
and so retards the speed of the pendulum. Be¬ 
tween these two extremes is a point which is neither 

quickened nor imped¬ 
ed by the rest, but 
moves in the same 
time that it would if 
it were a particle 
swinging by an imag¬ 
inary line. This point 
is called the centre of 
oscillation. It lies a 
little below the centre 
of gravity. In Fig. 21 
is shown an apparatus 
containing pendulums 
of different shapes, but 
all having the same 
absolute length. If 
they are started to¬ 
gether, they will im- 

























GRA VITA TION. 


6l 


mediately diverge, no two vibrat- Fig. 22 . 
ing in tlie same time. As pendu¬ 
lums, they are not of the same 
length. 

The centre of oscillation is found 
by trial. It has been discovered 
that the point of suspension and 
the centre of oscillation are inter- ii / 
changeable. If, therefore, a pen¬ 
dulum be inverted, and a point 
found at which it will vibrate in 
the same time as before, this is 
known to be the centre of oscilla¬ 
tion; while the old point of sus¬ 
pension becomes the new centre of 
oscillation. 

The Pendulum as a Time-keeper. 

—The friction at the point of sus¬ 
pension, and the resistance of the 
air, soon destroy the motion of the 
pendulum and bring it to rest. The 
common clock is simply a machine 
for keeping up the vibration of the 
pendulum and counting its beats. IN 
In Fig. 22, E is the scape-wheel 
driven by the force of the clock- 
weight or spring, and m n the es¬ 
capement, moved by the forked arm A B, so that 
only one cog of the wheel can pass at each double 
vibration of the pendulum. In this manner the 






62 


NA TUBAL PHIL OSOPHY. 



oscillations are counted by the cogs on the wheel, 
while the friction and resistance are overcome by 
the action of the weight or spring.* As “ heat ex¬ 
pands and cold contracts,” a pendulum increases in 
length in summer and shortens in winter. A clock, 
Fig. 23 . therefore, loses time in summer and 
gains in winter. To regulate a com¬ 
mon clock, we raise or lower the pen¬ 
dulum-bob, L, by means of a nut v at 
the lower end of the rod. 

The compensation or gridiron pendu¬ 
lum, consists of several brass and 
steel rods, which are so connected that 
the brass, h, le, will lengthen upward 
and the steel, a, h, c , d, will lengthen 
downward, and thus the centre of oscil¬ 
lation will be unchanged by any varia¬ 
tion in temperature. The mercurial 
pendulum contains a cup of mercury 
which expands upward while the pen¬ 
dulum-rod expands downward, and 
* thus keeps the centre of oscillation 
stationary. 

Various uses of the Pendulum .— 1. 
Since the time of the vibration of a pen¬ 
dulum indicates the force of gravity, 
and since the force of gravity decreases as the square 



* The action of a clock is best shown by procuring the works 
of an old clock, and watching the movements of the various parts. 












GRAVITATION. 


63 


of the distance from the centre of the earth increases, 
we may, in this manner, find the semi-diameter of the 
earth at various places, and thus ascertain the figure 
of our globe. 2. Knowing the force of gravity at 
any point, the velocity of a falling body can be 
determined. 3. It may be used as a standard of 
measures. 4. Foucault devised an ingenious method 
of showing the revolution of the earth on its axis, 
founded upon the fact that the pendulum vibrates 
constantly in one plane. 

Practical Questions .— 1 . When an apple falls to the ground, how much 
does the earth rise to meet it? 2. What causes the sawdust on a mill-pond to 
collect in large masses ? 3. Will a body weigh more in a valley or on a moun¬ 
tain? 4. Will a pound weight fall more slowly than a two-pound weight? 
5. How deep is a well if it takes three seconds for a stone to fall to the bottom 
of it ? 6. Is the centre of gravity always within a body,—as, for example, a 

ring? fc ~^-If two bodies, weighing respectively 2 and 4 lbs., be connected by a 
rod 2 feet long, where is the centre of gravity? 8. In a ball of equal density 
throughout, where is the centre of gravity? 9. Why does a ball roll down 
hill ? 10. Why is it easier to roll a round body than a square one ? 11. Why 

is it easier to tip over a load of hay than one of stone? 12. Why is a pyramid 
the stablest of structures ? 13. When a hammer is thrown, on which end does 

it always strike ? 14. Why does a rope-walker carry a heavy balancing-pole ? 

15. What would become of a ball if dropped into a hole bored through the 
centre of the earth ? 1G. Would a clock lose or gain time if carried to the top 

of a mountain ? If carried to the North Pole ? 17. In the winter, would you 

raise or lower the pendulum-bob of your clock? 1 8. Why is the pendulum- 
hob always made flat? 19. What beats off the time in a watch? 20. 
What should be the length of a pendulum to vibrate minutes at the latitude of 
New York? Solution— (1 sec.) 2 : (GO sec.) 2 :: 30.1 in. : x = 2.2 + miles.^21. 
What should be the length of the above to vibrate half-seconds ? Quarter- 
seconds? Hours? 22. Two pendulums are respectively 1G and Gi inches 
in length. What is their proportionate time of vibration ? 23. Why, wheq 
you are standing erect against a wall, and a piece of money is placed between 
your feet, can yoa not stoop forward and pick it up ? 24. If a tower were 198 
feet high, with what velocity would a stone, dropped from the summit, strike 
the ground? 25. A body falls in 5 seconds; with what velocity does it strike 
the ground ? 26. How far will a body fall in 10 seconds ? With what velocity 
will it strike the ground? 27 . A body is thrown upward with a velocity of 
192 feet the first second; to what height will it rise? (This problem is to 
be solved as if it read, “How far must a body fall to gain a velocity of 192 
feet ?”) 28 . A ball is shot upward with a velocity of 256 feet; to what height 



64 


?\i 


NATURAL PHILOSOPHY. 


% 

I o'J 


r ' r7 



will it rise* ?YHow Ion" will it continue to ascend ?' 29 . Why do not drops of 
water, falling from the clouds, strike with a force proportional to the laws of 
falling bodies ? A ns.— Because they are so small that the resistance of the air 
nearly destroys their velocity. If it were not for this wise provision, a shower 
of rain-drops would be as fatal as one of Mini6 bullets. 30. Arc any two 
plumb-lines parallel ? 31. A stone let fall from a bridge Btrikes the water/-7*7 
in 3 seconds. What is the height ? 32. A stone falls from a church-steeple^ 
in 4 seconds. What is the height of the steeple? 33. How far would a body 
fall in the first second at a distance of 12,000 miles above the earth’s surface? 

34. A body, at the surface of the earth, weighs 100 tons ; what would be its joMj\ 
weight 1,000 miles above ? 35. A boy, wishing to find the height of a steeple, 

lets fly an arrow that just reaches the top and then falls to the ground. It is in j 
the air G seconds. Required the height? 36 . A cat let fall from a balloon 
reaches the ground in 10 seconds. Required the distance ? 3 7. In what time / (j ij j. 

will a pendulum 40 feet long make a vibration ? 38. Two meteoric bodies in 

space are 12 miles apart. They weigh respectively 100 and 200 lbs. If they 
should fall together by force of their mutual attraction, what portion of the 
distance would be passed over by each body ? 39. If a body weighs 2,000 lbs. 

upon the surface of the earth, what would it weigh 2,000 miles above ? 500 
miles above ? 40. At what distance above the earth will a body fall, the first 
second, 21V8 inches ? 41 . How far will a body fall in 8 seconds ? In the^th^ 
second ? In 10 seconds ? In the 30th second ? 


The ancient methods of keeping time were simple indeed. The sun-dial was 
doubtless the earliest device; afterward the clepsydra was employed. This 
consisted of a vessel containing water, which slowly escaped into a dish be¬ 
low. In this was a floating body which, by its height, indicated the lapse of 
time. King Alfred the Great, we read, used candles of a uniform size, six of 
which lasted a day. He surrounded these with cases of horn as a protection 
from currents of air. From a mere fancied derivation of this kind, some have 
spelled the word lantern, lanthorn. Clocks were used in Europe as early a9 
the 11th century. The application of the pendulum was made in the early part 
of the 17th century. The first clock made in England, about a. d. 1288, was 
considered of so much importance, that a high official was appointed to take 
charge of it. The clocks of the Middle Ages were extremely elaborate. They 
indicated the motions of the heavenly bodies ; birds came out and sang songs, 
cocks crowed and trumpeters blew their horns ; chimes of bells were sounded, 
and processions of dignitaries and military officers, in fantastic dress, 
marched in front of the dial and gravely announced the time of day. Watches 
were made at Nuremberg in the 15th century. They were styled Nuremberg 
eggs. In the 16th century they were in common use. Many were as small as 
the watches of the present day, ■while others were as large as a dessert-plate. 
They had no minute or second hand, and required winding twice per day. 
They were extremely cumbersome, containing about 800 pieces. In 165S, Dr. 
Hare invented the main-spring. This gave to watches the accuracy of the 
pendulum. Waltham watches have but 120 pieces in all. Chronometers are 
now made so perfect as not to vary a minute in six months. 







< 6.7 



MOTION. 

Motion is a change of place. Absolute motion is 
a change without reference to any other object. 
Relative motion is a change with reference to some 
other object. Rest also is either Absolute or Rela¬ 
tive. Ex.: We are in absolute motion with the 
earth as it flies through space; when we walk, we 
judge of our motion by the objects around us; a 
man on a steamer is in motion with regard to the 
shore, but at rest with reference to the objects on 
the deck of the vessel. Nothing is in absolute rest. 
Motion seems to be a law of Nature. Velocity is the 
rate at which a body moves. Force is that which 
tends to produce or destroy motion. 

Resistances to Motion. —The principal are, Fric¬ 
tion, Resistance of the air, and Gravity. (1) Friction 
is the resistance caused by the surface over which 
a body moves. It is of two kinds, sliding and rolling. 
If the surface of a body could be made perfectly 
smooth, there would be no friction; but in spite of 
the most exact polish, the microscope reveals 
minute projections and cavities. We fill these 
with oil or grease, and thus diminish friction. Fric' 



68 


NATURAL PHILOSOPHY. 


tion, between different bodies, varies curiously. 
Between like substances it is greater than between 
unlike. Friction is of great value in common life. 
Without it, nails, screws, and strings would be 
useless; engines could not draw the cars; we 
could hold nothing in our hands ; everywhere we 
would walk as on glassy ice. (2) Resistance of the 
air. The resistance which a body meets in passing 
through air or water is caused by the particles 
which it must displace. This increases according to 
the square of the velocity. Thus, if we wish to double 
our speed in running we must displace twice as much 
air, and in half the time; hence, the force must be 
quadrupled. (3) Gravity tends to draw all bodies 
to rest upon the earth. 

Momentum is the quantity of motion in a body. 
It is equal to the weight of the body multiplied by 
its velocity per second, expressed in feet. Ex.: A 
stone, weighing 5 lbs., thrown with a velocity of 20 
feet per second, has a momentum of 100 pounds. 

The striking force of a body is equal to its weight 
multiplied by the square of its velocity. (See p. 81.) 
Ex.: A bullet weighing 2 ounces, fired with a velocity 
of 1,400 feet per second, would strike with a force of 
245,000 lbs. Place a hammer on the head of a nail, 
and, though you push with all your might, you 
cannot stir it. Swing the hammer by the handle 
and let it fall upon the nail, and the blow will bury 
it to the head. Oil the other hand, a large body 


MOTION. 


69 


may be moving very slowly and yet have an im¬ 
mense momentum. An iceberg, with a scarcely 
perceptible motion, will crush the strongest man-of- 
war as if it were an egg-shell. Those who have 
stood on a wharf have noticed with what prodigious 
force large vessels grind against each other by the 
slow movement of the tide. Soldiers have thought 
to stop a spent cannon-ball by putting a foot against 
it, but have found its momentum sufficient to break 
a leg. 

Motion is not imparted instantaneously .—We press 
with all our strength against a large stone. At first it 
does not stir. But the motion is transmitted from 
the molecules we touch with our hands, particle by 
particle, until it reaches the whole body, and the 
stone yields. A horse will pull at a heavy load for 
some moments before he starts it; if he should 
spring forward suddenly, he would be likely to break 
his harness. It is said that it would require a half 
minute for a force applied at one end of a mile of 
railroad-iron to move the last rail. A stone thrown 
against a pane of glass shatters it; but a bullet fired 
through it will only make a clean, round hole. The 
reason is, that the hole is made and the bullet gone 
before the motion has time to pass into the sur¬ 
rounding particles. A tallow candle may be fired 
through a board, because it pierces it so quickly that 
the particles have no time to yield. Its slight cohe¬ 
sion, multiplied by its velocity, is greater than the 
cohesion of the board. 


7o 


NATURAL PHILOSOPHY. 


1st Law of Motion. —A body once set in motion 
tends to move forever in a straight line . This is but 
another statement of the property of inertia, of 
which we have already spoken. There is a curious 
illustration of it seen in the swinging of a pendulum. 
A pendulum, made to vibrate with the least possible 
friction, is placed under the receiver of an air-pump. 
The more perfectly the air is exhausted the longer 
it will vibrate. In the best vacuum we can produce, 
it will swing for twenty-four hours. It is supposed 
that if all the “ resistances to motion” were removed, 
the pendulum would vibrate forever. Philosophers 
can explain this only on the supposition that a body, 
once started, tends to move forever in a straight 
line. For reasons which are obvious, no experiment 
can be performed which will directly prove the law. 
We can see the principle, however, in combination 
with the second law of motion. 

2d Law of Motion.— A force acting upon a body in 
motion or at rest , produces the same effect , whether it 
acts atone or with other forces. 

All bodies upon the earth are in constant motion, 
and yet we move anything with the same ease that 
we should, were the earth at rest. We throw a stone 
directly at an object and hit it, yet, within the 
second, the mark has gone forward many feet.* A 
ball thrown up into the air with a force that would 
cause it to rise 50 feet, will ascend to that height 
whatever horizontal wind may be blowing at the time. 
If a cannon-ball or a bullet be thrown horizontally, it 

* The earth moves forward in its orbit about the sun at the rate of 18 milea 
per second. See Fourteen Weeks in Astronomy, p. 106. 


MOTION. 


71 


will fall just as fast and strike tlie earth just as 
soon as if dropped to the ground from the muz¬ 
zle of the gun. In Fig. 24, D is an arm driven 
by a wooden 
spring, E, and 
turning on a 
hinge at C. 

At D is a hol¬ 
low contain¬ 
ing a bullet 
so arranged 
that when the 
arm is sprung, 
it will-throw the ball in the line F K. At F is a 
similar ball, supported by a thin slat, G, and so 
arranged that the same blow which throws the 
ball D, will let the ball F fall in the line F H. 
It will be found that the two balls will strike the 
floor together. This holds true, no matter how far 
the ball D may be thrown. "We here see that the 
force of gravity produces the same effect whether it 
acts alone or in combination with another force. 

Compound Motion. —Let a ball at A be struck by 
a force which would Fig. 25. 

drive it in the direc¬ 
tion A B, and also 
at the same instant 
by another which 
would drive it toward D; the ball will move in the 
direction A C. The figure A B C D is termed the 



Fig. 24. 






72 


NATURAL PHILOSOPHY. 


“ Parallelogram of the Forces,” and the diagonal 
A C the “ Resultant.” 

*** Composition of Forces.— Whenever a body is 
Fig. 26. acted upon by 

two forces, we 
draw lines rep¬ 
resenting these 
directions, and 
mark distances 
A D and A B, 
whose lengths 
represent their 
comp arative 
velocities. We 
next complete 
the parallelo¬ 
gram and draw the diagonal A C, which denotes the 
resultant of these forces, or the direction in which the 
body will move. If more than two forces act, we 
find the resultant of two, then of that resultant and 
a third force, and so on. 

Illustrations .—We have many illustrations of com¬ 
pound motion in common life. A person wishes to 
row a boat across a swift current. It carries him 
down stream. He steers, therefore, toward a point 
above that which he wishes to reach, and so goes 
directly across.—While riding on a car, we throw a 
stone at some object at rest. The stone, having the 
motion of the train, strikes just as far ahead of the 
object as it would have gone had it remained on the 



COMPOUND MOTION. 


73 


train. In order to hit the mark, we should have 
aimed a little back of it.—The circus-rider wishes, 
while riding his horse at full speed, to jump through 
a hoop suspended before him. He simply springs 
directly upward. He goes forward by the motion 
which he had when he leaped from the horse. The 
resultant motion carries him through the hoop and 
he alights upon the saddle on the other side.—A 
person riding in a coach drops a cent to the floor. It 
falls in a vertical line and strikes where it would were 
the coach at rest.—A bird, beating the air with both 
its wings, flies in a direction between that of the 
twp. 

Resolution of Forces. 

—This is the reverse of 
the “ Composition of 
Forces.” It consists in 
finding what two forces 
are equivalent to a given 
force. It is attained by 
drawing a parallelogram 
having the given force 
for a diagonal. Ex.: 

There is a wind blowing 
from the West against 
G H, the sail of a vessel 
going North. We can resolve the wind-force B D 
into the two forces B E and B C. The former, 
blowing parallel to the sail, is of no use; the latter 
is perpendicular to it, and hence tends to drive the 


Fig. 27. 

N 



6 




NATURAL PHILOSOPHY. 


74 

vessel before it in a northeast direction. Again, 
resolving B D in Fig. 28, which represents the 
vertical force B C in Fig. 27, 
we find that it is equivalent 
to two forces, B E and B C. 
The former pushes the vessel 
sidewise, but it is mainly coun¬ 
teracted by the shape of the 
keel and the action of the rud¬ 
der. The latter is parallel to 
the course of the ship, and hur¬ 
ries it along northward. By 
shifting the rigging, one vessel 
will sail into harbor while an¬ 
other is sailing out, both 
driven by the same 
wind. Figs. 29 and 30 
show how, by twice re¬ 
solving the force of the 
wind from the West, as 
in the last figures, when 
the sail G H is placed 
in the new position, we 
have (Fig. 30) a force, 
B C, which drives the 
vessel southward. If 
a vessel should wish to 
sail directly W. against this wind we have supposed, 
it would tack alternately NW. and SW. In this way 
it could go almost into the “teeth of the wind.” 








COMPOUND MOTION. 


75 


In a similar manner we may resolve the three 
forces which act upon a kite—viz., the pull of the 
string, the force of the wind, Fig. 30. 

and its own weight. In Fig. ^ 

27 let G H represent the face 
of the kite. We can resolve 
B D, the force of the wind, 
into B C and B E. We 
next resolve B D, in Fig. 

28, which corresponds to 
B C in Fig. 27, into B E 
and B C. We then have a 
force, B C, which overcomes 
the weight of the kite and 
also tends to lift it upward. $ 

The string pulls in the direction D B, perpendicularly 
to the face. The kite obeys neither one of these 
forces but both, and so ascends in a direction, D G, 
between the two. It is really drawn up an inclined 
plane by the joint force of the wind and the string. 

A canal-boat drawn by horses is acted upon by a 
force which tends to bring it to the bank. This 
force may be resolved into two, one pulling toward 
the tow-path, and the other directly ahead. The 
former is counteracted by the shape of the boat and 
the action of the rudder; the latter draws the boat 
forward. 

Circular Motion is a variety of compound motion 
produced by two forces called the Centrifugal and 
the Centripetal . The former ( centrum , the centre, and 





NATURAL PHILOSOPHY. 


fngio , to flee) tends to drive a body from the centre. 


The latter ( centrum , the centre, and peto, J 



tends to draw a body toward the centre. 

The motion of the heavenly bodies presents the 


grandest illustration of the operation of these forces. 


The earth, when first formed, we may suppose, was 
hurled into space from the hand of the Creator with 
a force which would send it along the line B C in 
Fig. 31. According to the law of inertia it would- 
never lose this force, but would continue to move 
forever in a straight line. Being attracted, how¬ 
ever, by the sun in the direction B S, it passes along 


Fig. 31. 


c 



- © 


E 


© 


the line B D, which is the resultant of these two 
forces. Should the earth ever lose its own motion, 
it would fall into the sun and feed that central fire. 
Should the attraction of the sun cease, it would fly 
off with headlong speed into the icy, cheerless regions 
of space. 




CIRCULAR MOTION. 


77 


Examples abound in common life. Water flies 
from a grindstone on account of the centrifugal 
force produced in the rapid revolution, which over¬ 
comes the force of adhesion. In factories, grind¬ 
stones are sometimes revolved with such velocity 
that the centrifugal force overcomes the force of 
cohesion , and the ponderous stones fly into fragments. 
A pail full of water may be whirled around so rap¬ 
idly that none will spill out, because of the centrif¬ 
ugal force which overcomes the force of gravity. 
When a horse is running around a small circle he 
bends inward to overcome the centrifugal force. 

The rapid revolution of the earth on its axis tends 
to throw off all bodies headlong into space. As this 
force acts contrary to that of gravity, it diminishes 
the weight of all bodies at the 
equator, where it is greatest, 

'zfc. It also tends to drive the 
water on the earth from the 
poles toward the equator, and 
in consequence to heap it 
up in the equatorial region 
of the ocean. Were the ve¬ 
locity of the earth’s rotation to 
diminish, the water would run 
back toward the poles, and tend 
to restore the earth to a spherical form. This in¬ 
fluence is well illustrated by the apparatus shown 
in the figure. The hoop is made to slide upon its 
axis, and if it is revolved rapidly it will assume an 





78 


NATURAL PHILOSOPHY. 


oval form, bulging out more and more as the velocity 
is increased.* X 

Third Law of Motion.— Action is equal to reaction, 
and in the contrary direction . A bird in flying beats 
the air downward, but the air reacts and supports 
the bird. The boatman pushes with his pole against 
the dock, and by the reaction his boat is driven from 
the shore. The oarsman strikes the water backward 
with the same force, that his boat moves forward. 
The swimmer kicks with his feet against the water, 
which reacts and sends him ahead. The powder in 
a gun explodes with equal force in every direction, 
driving the gun backward and the ball forward, each 
with the same momentum. Their relative velocities 
vary with their respective weights ; the heavier the 
gun the less will the recoil be noticed. When we 
spring from a boat, unless we are cautious, the re¬ 
action will drive it away from the shore. When we 
jump from the ground, we push the earth from us, 
while it reacts and pushes us away from it; we sep¬ 
arate from each other with equal momenta, and our 
velocity is as much greater than that of the earth 
as we are lighter. We cannot jump from a soft sur¬ 
face, because it yields; but a spring-board, which 
reacts more promptly, aids us. We walk by reason 


* This apparatus is always accompanied by a variety of objects 
which may be used to illustrate very beautifully the principle 
that all bodies tend to revolve about their shortest diameters. 
This is an assurance that the earth will never change its axis of 
rotation while it retains its present form. 



REFLECTED MOTION. 


79 


Fig. 33. 



A BCDE F 


of the reaction of the ground on which we tread. 
Thus at every step we take, we cause the earth to 
move.* The apparatus shown in the figure consists 
of ivory balls so hung as to readily 
vibrate. If a ball be let fall from one 
side it strikes the second ball, which 
reacts with an equal force and stops the 
motion of the first, but transmits the 
motion to the third; that acts in the same manner, 
and so on through the series, each acting and re¬ 
acting until the last ball is reached; this reacts and 
then bounds off, rising as high as the first ball fell 
(except the loss caused by friction). If two balls be 
raised, two will fly off at the opposite end; if two 
be let fall from one side and one from the other, 
they will respond alternately from either side. 

Reflected Motion. —This is pro¬ 
duced by the reaction of any surface A 
against which an elastic body is B 
thrown. If a ball be thrown in the 

c 

directionO B against the surface AC, 
it will rebound in the line B R. The angle O B P 


Fig. 34. 



* No force in nature can be wasted. It must accomplish some¬ 
thing. “ A blow with a hammer moves the earth. A boy could 
in time draw the largest ship across the harbor in calm weather.” 

“ Water falling day by day 
Wears the hardest rock away.” 

Statues are worn smooth by the constant kissing of enthusiastic 
worshippers. Stone steps are hollowed by the friction of many 
feet. The ocean is filled by small drops which fall from the 
clouds. We may notice none of these forces singly, but their 
effects in the aggregate startle us. 










8o 


NATURAL PHILOSOPHY. 


(the angle of incidence) will be equal to the angle 
PBR (the angle of reflection). 

Motion in a curved line.— Whenever two or more 
instantaneous forces act upon a body, the resultant 
is a straight line. When one is instantaneous, and 
the other continuous, it is a curved line. When a 
body is thrown into the air, unless it be in a ver¬ 
tical line, it is acted upon by the instantaneous 
force of projection and the continuous force of 
gravity, and so passes through a line which curves 
toward the earth. 

Perpetual Motion. —Nothing can be more utterly 
impracticable than to make a machine capable of 
perpetual motion. No machine can produce power; 
it can only direct that which is applied to it. In all 
machinery there is friction; this must ultimately 
exhaust the power and bring the motion to rest. 
These principles show the futility of all such at¬ 
tempts. 


^Practical Questions .—1. Can a rifle-ball be fired through a handkerchief 
suspended loosely from one corner ? 2. A rifle-ball thrown against a board 
standing edgewise, will knock it down ; the same bullet fired at the board, will 
pass through it without disturbing its position. Why is this ? 3. Why can 
a boy skate safely over a piece of thin ice, when, if he should pause, it would 
break under him directly? 4. "Why can a cannon-ball be fired through a door 
standing ajar, without moving it on its hinges? 5. Why can we drive on 
the head of a hammer by simply striking the end of the handle ? G. Suppose 
you were on a train of cars moving at the rate of 30 miles per hour; with what 
force would you be thrown forward if the train were stopped instantly ? 7 . In 

what line does a stone fall from the masthead of a vessel in motion ? 8. If a 
ball be dropped from a high tower it will strike the ground a little east of a ver¬ 
tical line. Why is this ? 9. It is stated that a suit was once brought by the 
driver of a light wagon against the owner of a coach for damages caused by a 
collision. The complaint was that the latter was driving so fast that when the 
two carriages struck, the driver of the former was thrown forward over the 
dashboard. On trial he was nonsuited, because his own evidence showed him 


MOTION. 


81 


to be the one who was driving at the unusual speed. Explain. 10. Suppose 
a train moving at the rate of 30 miles per hour: on the rear platform is a cannon 
aimed parallel to the track and in a direction precisely opposite to the motion of 
the car. Let a ball be discharged with the exact speed of the train; where 
would it fall ? 11. Suppose a steamer in rapid motion, and on its deck a man 

jumping. Can he j ump farthei by leaping the way the boat is moving or in the 
opposite direction? 12. Why is a “ running jump” longer than a standing 
one? 13. If a stone be dropped from the masthead of a vessel in motion, 
will it strike the same spot on the deck that it would if the vessel were at 
rest? 14. Could a party play ball on the deck of the Great Eastern when 
steaming along at the rate of 20 miles per hour, without making allowance for 
the motion of the ship ? 15. Since action is equal to reaction, why is it not 

as dangerous to receive the “ kick” of a gun as the force of the bullet ? 16. 

If you were to jump from a carriage in rapid motion, would you leap directly 
toward the spot on which you wished to alight ? 17. If you wished to shoot a 

bird in swift flight, would you aim directly at it? 18. At what parts of the 
earth is the centrifugal force least ? 19. W'hat causes the mud to fly from the 

wheels of a carriage in rapid motion? 20. What proof have we that the 
earth was once a soft ma ss f\) 21. On a curve in a railroad, one track is always 
higher thap the other, mtyis.this? 22. What is the principle of the sling? 
23. The mouth of the Mississippi River is about 2f miles farther from the cen¬ 
tre of tire earth than its source. In this sense it may be said to “ run up hill.” 
What causes this apparent opposition to the attraction of gravity? 24. Is it 
action or reaction that breaks an egg, when I strike it against the table? 25. 
Was the man philosophical who said that it “ was not the falling so far, but 
the stopping so quick, that hurt him?” 26. If one person runs against an¬ 
other, which receives the greater blow? 27. Would it vary the effect if the 
two persons were running in opposite directions ? In the same direction ? 
28. Why cannot you fire a rifle-ball around a hill ? 29. Why is it that a heavy 
rifle ‘’kicks” less than a light shot-gun? 30. A man on the deck of a large 
vessel draws a small boat toward him. How much does the ship move to meet 
the boat? 31. Suppose a string, fastened at one end, will just support a 
weight of 25 lbs. at the other. Unfasten it, and let two persons pull upon it in 
opposite directions. How much can each pull without breaking it ? 32. Can 
a man standing on a platform-scale imike himself lighter by lifting up on him¬ 
self? 33. Why cannot a man lift himself by pulling up on his boot-straps ? 
34. If, from a gun placed vertically, a ball were tired into perfectly still air, 
where would it fall? 35. With what momentum would a steamboat weigh¬ 
ing 1,000 tons, and moving with a velocity of 10 feet per second, strike against 

a sunken rock?-(On page 68, we found that a constant force which tends to 

overcome a continued resistance, like that of air or water, must be as the 
square of the velocity. This is termed its living force, or vis viva. This law 
holds good only in starting bodies from a state of rest and in low velocities. 
At high rates of speed less force is required. The comparative striking force 
is, however, always as the square of the velocity.)-36. With what mo¬ 

mentum would a train of cars weighing 100 tons, and running 10 miles pet 
hour, strike against an obstacle ? 37. What would be the comparative strik¬ 

ing force of two hammers, one driven with a velocity of 20 feet per second 
and the other 10 feet ? 




* * 































































































































































































































































THE ELEMENTS OF MACHINERY. 

These are the simple machines to which all ma¬ 
chinery can be reduced. The watch, with its complex 
system of wheel-work, and the engine, with its belts, 
cranks, and pistons, are only various modifications 
of some of the six elementary forms—viz., the lever, 
the wheel and axle, the inclined plane, the screw, the 
wedge, and the pulley. These six may be still further 
reduced to the lever and inclined plane. They are 
termed powers, but do not produce force; they are 
only methods of applying and directing ii# They 
also enable us to use the forces of Nature, such as 
wind, water, and steam. The work done by the 
power is always equal to that done by the weight. 
The law of all mechanics is— 

The power multiplied by the distance through ivhich 
it moves , is equal to the weight multiplied by the distance 
through ivhich it moves. Thus 1 lb. of power moving 
through 10 feet = 10 lbs. of weight moving through 
one foot. 

The Lever is a bar turning on a pivot. The force 
Used is termed the power (P), the object to be lifted 
the weight (W), the pivot on which th<e lever turns 


86 


NATURAL PHILOSOPHY. 


the fulcrum (F), and the parts of the lever each side 
of the fulcrum the arms. 

The three classes of levers. —I. Power at one end, 
weight at the other, and fulcrum between. II. 


Fig. 5 


Fig. 35. 

F 


Fig. 37. 




r 

w' 


Fig. 38. 


Power at one eiid, fulcrum at the other, and weight 
between. III. Weight at one end, fulcrum at the 
other, and power between. 

\st Class of Lever .—We wish to lift a stone. We 
put one end of a handspike 
under the stone, and resting the 
bar on a block at F, we bear 
down at P. A pump-handle is 
a lever of the first class. The 
hand is the P, the water lifted 
the W, and the pivot the F. A pair of scissors is a 
double lever of the same class. The cloth to be cut 
is the W, the hand the P, and 
the rivet the F. 

2d Class .—We may also 
raise the stone, as in Fig. 39, 
by resting one end of the 
lever on the ground, which acts as a fulcrum, and 











MECHANICAL POWERS. 


87 

lifting up on ^the bar. An oar is a lever of the 
second class. The hand is the P, the boat the W, 
and the water the F. 

3 d Class .—The treadle is a lever of the third class. 
The end, C, resting on the ground is the F, the foot 


Fig. 40. 



is the P, and the force is transmitted by a rod to the 
W, the reel above. In the fishing-rod, one hand is 
the F, the other the P, and the fish the W. 

Laiv of Equilibrium .—The lever is in equilibrium 
when the arms balance each other. The distance 
through which the P and the W move depends upon 
the comparative length of the arms. Let Pd repre¬ 
sent power’s distance from the F, and W d weight’s 
distance; then if Pd is twice Wd, the power will 





88 


NATURAL PHILOSOPHY. 


move twice as far as the weight. Substituting these 
terms in the law of Mechanics, we have 

P X Pd = W X W d, or P : W :: Wd : Pd. 

In the first and second classes, as ordinarily used, 
we gain power and lose time; in the third class we 
lose power and gain time. 

The Steelyard is a lever of the first class. The 


Fig. 41. 



power is at E, the fulcrum at C, and the weight at 
D. If the distance from the pivot of the hook D to 
the pivot of the hook C is one inch, and from the pivot 
of the hook C to the notch where E hangs is 12 inches, 
then a 1-lb. weight at E will balance 12 lbs. at W. 
If the steelyard be reversed, as in Fig. 42, then the 
distance of the fulcrum from the W is only 1 as 
great, and the same weight at E will balance 48 lbs. 










MECHANICAL POWERS. 

at D. Two sets of notches on opposite sides of the 
bar correspond to these two positions. 


Fig. 42. 



The Arm is a lever of the third class. The muscle 
(Physiology, p. 48) is attached- to the bone of the 
forearm, at a distance of about two inches from the 
elbow joint, while from the centre of the palm of the 
hand to the same point is about 13 inches. Hence 
W d = 13 inches and Vd = 2 inches. Therefore the 
force exerted by the muscle must be over six times 
the weight to be lifted by the hand. What we thus 
lose in power, we gain in the speed of the motion. 
We desire to perform quick movements with our 
hands, and so they are wisely and expressly com 
trived to meet our wants. 

Bent Lever.—In the hammer, when used to dra^ 








9 o 


NATURAL PHILOSOPHY 



a nail, we have a good illustration of a bent lever. 

The real length of the arms is that of 
the straight lines which correspond to 
the direction in which the power and 
weight act with reference to the fulcrum. 

The Compound Lever consists of sev¬ 
eral levers so connected that the short 
arm of the first acts on the long arm of the second, 
and so on to the last. If the distance of A from the 
F be four times that of B, then a power of 5 lbs. at 
A will lift a W of 20 lbs. at B. If the arms of the 
second lever are of the same comparative length, 
then a power of 20 lbs. at C will lift 80 lbs. at E. 
In the third lever, a power of 80 lbs. at D will, in 
the same proportion, lift 320 lbs. at G. Thus, with 
Fi „ 44 this system of three 

levers, a power of 5 
lbs. will balance a 
weight of 320 lbs. In 
order, however, to raise the weight one foot, the 
power must pass through 64 feet. Hay-scales are 
constructed upon the principle *of 
the compound lever. 

The Wheel and Axle is a modi¬ 
fication of the lever. The wind¬ 
lass used for drawing water from 
a well, is a common form. The 
power is applied at the handle, the 
bucket is the W, and the F is the 
axis of the windlass. The long arm of the lever is 


« u ^ A 

i 

E 

B 

ll 


Fig. 45. 














MECHANICAL POWERS. 


9 1 


the length of the handle, and the short arm is the 
semi-diameter of the axle. This is seen very clearly 
in the cross-section shown 
in Fig. 45, where 0 is the 
F, 0 A the long arm, and 
O B the short arm. In 
Fig. 46, instead of turning 
a handle we take hold of 
pins inserted in the rim of 
a wheel. Fig. 47 represents 
a capstan used on board 
of vessels for weighing the 
anchor. The power is applied by means of hand¬ 
spikes which radiate outward from the axle. Fig. 48 
shows a form of the capstan Fig. 47. 

often used in moving build¬ 
ings, in which a horse fur¬ 
nishes the power. The wheel 
and axle has the advan¬ 
tage that it is a kind of per¬ 
petual lever. We are not obliged to prop up the W and 
readjust the lever, but both arms work continuously. 

Law of Equilibrium .—By turning the handle or 
wheel around once, the rope will be wound once 
around the axle and the W be lifted that distance. 
Applying the law of Mechanics, we see that the 
Power X* the circumference of the Wheel = the 
Weight X circumference of the axle; or, as circles 
are proportional to their radii, 

P : W : : Radius of the Axle : Radius of the Wheel. 



Fig. 46. 



$2 


NA TURAL PHILOSOPHY. 


If the radius of an axle be 6 inches, and the radius 


Pig. 48. 



of the wheel 24 inches, then the weight will exceed 
the power four times. 

Fig. 49 . Wheelivork consists of a series 

of wheels and axles which act 
upon each other on the princi¬ 
ple of a compound lever. The 
cogs on the circumference of the 
wheel are termed teeth, on the 
axle leaves, and the axle itself a 
pinion. If the radius of the wheel F is 12 inches, 
and that of the pinion 2 inches, then a power of 1 lb. 
will apply a force of 6 lbs. to the second wheel E. 
If the radius of this is also 12 inches, then the 
second wheel will apply a force of 36 lbs. to the 
third wheel. This, acting on its axle, will balance a 
W of 216 lbs. In order, however, to lift this amount, 
according to the principle already named, the weight 
will only pass through of the distance of the 












MECHANICAL POWERS. 


93 


power. We thus gain power and lose speed. If we 
wish to reverse this we can apply the power to the 
axle, and, with a correspondingly heavy pow r er, 
gain speed. This is the plan adopted in factories, 
where a heavy water-wheel furnishes abundance of 
power, and spindles or other swift machinery are to 
be turned very rapidly. 

The Inclined Plane. —If we wish to lift a heav 
cask into a wagon, we rest one end of a plank on 
the wagon-box and the other on the ground. We 
can then roll the cask up the inclined plane thus 


Fig. 50. 



formed, when we could not have lifted it directly. 
When roads are to be made over steep hills, they 
are sometimes constructed around the hill, like the 
thread of a screw, or in a winding manner, as shown 






94 


NATURAL PHILOSOPHY. 


in Fig. 50. The road from Callao to Lima, in 
South America, is said to be one of the longest and 
best-made inclined planes in the world. It is six 
miles in length, and the total rise is 511 feet. 
Stairs are inclined planes with steps cut in them to 
facilitate their ascent. 


Law of Equilibrium. 


Fig. 51. 


n In Fig. 51 we see that 
the power must descend 
a distance equal to A C 
in order to elevate the 



weight to the height C B. Applying the law of 
Mechanics, we have P X length of the inclined 
plane = W X height of inclined plane; hence, 


P : W : : height of inclined plane : length of inclined plane. 


Thus, if we roll a barrel of pork, weighing 200 lbs., 
up a plane 12 feet long and 3 feet high into a wagon, 
we have x — 50 lbs. : 200 lbs. :: 3 feet : 12 feet. In 
this case we lift only 50 lbs., or { of the barrel, but 
we lift it through four times the space necessary if 
we could have raised it directly into the wagon. 
We thus lose speed and gain power. The longer 
the inclined plane, the greater the load we can lift, 
but the longer it will take to do it. If a road as¬ 
cends one foot in 100 feet, then a horse drawing up 
a wagon has to lift only T L> of the load, besides 
overcoming the friction. A body rolling down an 
inclined plane acquires the same velocity that it 
would in falling the same height perpendicularly. 



mechanical powers. 


95 


A train descending a grade of one foot to 100 
feet tends to go down with a force equal to of 
its weight. Near Lake Lucerne, Switzerland, is a 
valuable forest of firs on the top of an almost inac¬ 
cessible Alpine mountain. By means of a wooden 
trough, the trees are conducted into the water 
below, a distance of eight miles, in as many minutes. 
One standing near hears a roar as of distant thunder, 
and the next instant the descending tree darts past 
him and plunges downward out of sight. The force 
with which it falls is so prodigious, that if it jumps 
out of the trough it is dashed to pieces. 

The Screw consists of an inclined plane wound 
around a cylinder. The inclined plane forms the 
thread, and the cylinder the 
body. It works in a nut which 
is fitted with reverse threads 
to move on the thread of the 
screw. The nut may turn on 
the screw, or the screw in the 
nut. The power may be ap¬ 
plied to either as desired, by 
means of a wrench or a lever. The screw is used in 
presses for squeezing oil and juices from apples, 
grapes, rapeseed, linseed, sugar-cane, etc.; for copy¬ 
ing letters, for coining money; in vises and in rais¬ 
ing buildings. 

Law of Equilibrium .—When the power is applied 
at the end of a lever, it describes a circle of which 
the lever is the radius. The distance through which 





NATURAL PHILOSOPHY. 


96 

the power passes, is the circumference of this circle; 
and the height to which the weight is elevated at 
each revolution of the screw, is the distance between 
two of the threads. Applying now' the law of 
Mechanics, we have P X circumference of circle = 
W X interval between the threads; hence, 

P : W r :: interval : circumference. 

The power of the screw may be increased by length¬ 
ening the lever, or by diminishing the distance be¬ 
tween the threads. 

The Wedge usually consists of two inclined planes 
placed back to back. It is used for 
splitting logs of wood and blocks of 
stone; for lifting vessels in the dock; 
and, in oil-presses, for squeezing. 
Chimneys which have leaned over, 
have been righted by wedges driven 
in on the lower side. Nails, needles, 
axes, etc., are constructed on the principle of the 
wedge. 

The Law of Equilibrium is, in theory, the same as 
that of the inclined plane—viz., 

P : W :: thickness of wedge : length of wedge. 

In practice, however, this by no means accounts for 
its prodigious power. Friction, in the other mechani¬ 
cal powers, materially diminishes their efficiency; in 
this it is essential, since, without it, after each blow 
the wedge would fly back and the whole effect be 
lost. Again : in the others, the power is applied as 





MECHANICAL POWERS. 


97 


Fig. 65. 


a steady force: in this it is a sudden blow, and is^ 
equal to the momentum of the hammer. 

The Pulley is simply another form of the lever 
which turns about a fixed axis or fulcrum. It con¬ 
sists of a wheel, within the grooved edge of which 
runs a cord. 

When we wish to transmit force from one point to 
another, we may do so either by 'pushing with a rigid 
bar, or by pulling with a flexible cord. The advan¬ 
tage of the latter method is, that we may 
at the same time change the direction 
of the force. This is accomplished by a 
single fixed pulley , as in Fig. 55. Here 
there can be no gain of power or of speed, 
as the hand F must pull down as much 
as the weight W, and both move with 
the same velocity. It is simply a lever 
of the first class with equal arms. But its use is 
seen when we remember how, by means of it, a man 
standing on the ground 
hoists a flag to the top of 
a lofty pole, and thus 
avoids the trouble and 
danger of climbing up with 
it. Two fixed pulleys, 
arranged in the manner 
shown in Fig. 56, enable 
us to elevate a heavy load 
to the upper story of a 
building by horse-power. 



Fig. 56. 



5 













NATURAL PHILOSOPHY. 


98 



A- 




Movable Pulley .—A form of the single pulley, where 
it moves with the W, is represented in Fig. 57. In 
Fig. 57 . this, one-half of the barrel is sustained by 
the hook F, while the hand lifts the other. 
Since, then, the power is only one-half the 
weight, it must move through twice the 
space; in other words, by taking twice the 
time, we can lift twice as much. Here 
power is gained and time lost. 

We may also explain the action of the 
single movable pulley by Fig. 58, in which 
A represents the F, B the W acting in 
the line O K, and B the P acting in the 
S line B P. This is a lever of the second 
class; and as B is twice as far from A 
as O is, the power is only one-half 
- 1 the weight. 

Combinations of Pulleys. —1. In Fig. 59, we have 
the W sustained by three cords, 
each of which is stretched by 
a tension equal to the P, hence 
1 lb. of power will balance 3 lbs. 
of weight. 2. In Fig. 60, the 
power will in the same manner 
sustain a W of 4 lbs., and must 
descend 4 inches to raise the 
W one inch. 3. In the cord 
marked 1, 1 (Fig. 61), each 
part has a tension equal to P; 
and in the cord marked 2, 2, 


Fig. 59. 


Fig. 60. 
























MECIIA NIC A L PO WEBS. 


99 


eacli part lias a tension equal to 2P, and so on with 
the other cords. The sum of the tensions acting 


Fig. 61. 


Fig. 62. 


W is 16, hence W = 16 P. 

Fig. 62 represents the 
ordinary “ tackle-block” 
used by mechanics. 

Law of Equilibrium .— 

In all combinations of 
pulleys, nearly one-half 
the effective force is lost 
by friction.* In most of 
the forms in use, the W 
is equal to the P multi¬ 
plied by twice the num¬ 
ber of movable pulleys. 

t actical Questions . — 1 . Describe the rutlder of a boat as a lever ; a door; 
a door latch; a lemon-squeezer; a pitchfork; a spade; a shovel; a sheep- 
shears ; a poker; a pair of tongs : a balance; a pair of pincers; a wheelbarrow; 
a man pushing open a gate with his hand near the hinge; a chopping-knife 
(Fig. 63); a sledge-hammer, when the arm is swung 
from the shoulder; a nut-cracker. 2 . Show the 
change that occurs from the second to the third 
class of lever, when you take hold of a ladder at one 
end and raise it against a building. 3. Why is a 
pinch from the tongs near the hinge more severe 
than one near the end? 4. Two persons are carry¬ 
ing a weight of 250 lbs., hanging between them from a pole 10 feet in-' 
length. Where should it be suspended so that one will lift only 50 lbs. ? 
5. In a lever of the first class, 6 feet long, where should the F be place 
so that a power of 1 lb. will balance a W of 23 lbs. ? 6. What power would 
he required to lift a barrel of pork with a windlass whose axle is on 
loot in diameter, and handle 3 feet long ? 7. What sized axle, with a wheel 
6 feet in diameter, would be required to balance a weight of one ton by a 



Fig. 63. 




* The work lost is not destroyed, for this is an impossibility. 
No force nor matter has been destroyed, so far as we know, 
since the creation of the world. The force is converted into other 
forms—heat, electricity, etc., according to the principle of the 
correlation of forces, p. 31G. 




















y 


NATURAL PHILOSOPHY. 


100 

power of 100 lbs. What number of movable pulleys would be required 
to lift a W of 200 lbs. by means of a power of 25 lbs. How many lbs. could 
be lifted with a system of 4 movable pulleys and one fixed pulley to change 
the direction of the force, by a power of 100 lb 0. What weight could be / (yWj 
lifted with a single horse-power* acting on the system of pulleys shown in' > ^ 
Fig. 62 (tackle-block) ? 11. What distance should there be between the 

threads of a screw in order that a Pof 25 lbs. acting on a handle three feet long, 
may lift a ton weight ? 12. How high would a P of 12 lbs., moving 16 feetxil- q 

along an inclined plane, lift a W of 96 lbs. ? 13.1 wish to roll a barrel o£-7 W «-=. 

flour into a wagon, the box of which is four feet from the ground. I can lift but-' ^ 

24 lbs. How long a plank must I get ? 14. The “evener” of a pair of whiffle-^(/\\ 0^' 
trees is 3 feet 6 inches in length ; how much must the whiffletree be moved to 
give one horse an advantage of one-third over the other ? 15. In a set of 

three-horse whiflletrees, having an “ evener" 5 feet in length, at what point 
should the plough-clevis be attached that the single horse may draw the same 
as each of the span of horses? At what point to give him one-quarter ad vati— N 
tage ? 16. What weight can be lifted with a power of 100 lbs. acting ofr^a 

screw having threads one-quarter of an inch apart,and a lever handle 4 feet long ? 

1 7 . What is the object of the big balls always cast on the ends of the handle 

of the screw used in presses for copying letters ? 18. In a pair of steelyards 

2 feet long, the distance from the weight-hook to the fulcrum-hook is 2 inches ; 
how heavy a body can be weighed with a 1-lb. weight at the further end ? 

19. Describe the change from the first to the third class of levers, in the dif¬ 
ferent ways of using a pitchfork or spade. 20. Why are not blacksmiths' tongs 
and fire-tongs constructed on the same principle ? 21. In a lever of the third, 
class, what W will a P of 50 lbs. balance, if one arm is 12 feet and the other 

3 feet long ? 22. In a lever of the second class, what W will a F of 50 lbs. 

balance, with a lever 12 feet long, and W 3 feet from the F ? 23. In a lever 

of the first class, what W will a P of 50 lbs. balance, with a lever 12 feet long,, /)£/ 
and the F 3 feet from the W ? 24. In a wheel and axle, the P = 40 lbs., the ~ 

W = 360 lbs., and the diameter of the axle = 8 in. Required the circumference 
of the wheel. 25. Suppose,in a wheel and axle, the P= 20 lbs., the W = 240 
lbs., and the diameter of the wheel = 4 feet. Required the circumference of the 
axle? 26. Required, in awheel and axle, the diameter of the wheel, the 
diameter of the axle being 10 inches, the P 100 lbs., and the W 1 ton ? 27. 

What P would be necessary to sustain a W of 3,180 lbs. with a system of si.y 
movable pulleys, and a single rope passing over them all ? 28. How many 
movable pulleys would be required to sustain a W of 420 lbs., with a Pol 
210 lbs. ? 



* A horse-power is reckoned in Mechanics as a force which will lift 32.000 
lbs. one foot in one minute, without any assistance of machinery. 





*< ( 





:] restore of | irpiids and 


“ The waves that moan along the shore, 
The winds that sigh in blowing, 

Are sent to teach a mystic lore 
Which men are wise in knowing.” 









HYDROSTATICS. 


Hydrostatics treats of liquids at rest. Its prin¬ 
ciples apply to all liquids; but water, on account of 
its abundance, is taken as the type of the class, and 
all experiments are based upon it. 

I. Liquids transmit pressure equally in all 
directions.— This is the first Fi s- 64 - 

and most important law. As 
the particles of a liquid move 
freely among themselves, 
there is no loss by friction, 
and any force will be trans¬ 
mitted equally, upward, 
downward, and sidewise. 

Thus if a bottle be filled 
with water and a pressure of 
1 lb. be applied upon the 
cork, it will be communicated 
from particle to particle 
throughout the water. If 
the area of the cork be one 
square inch, the pressure upon any square inch of 





104 


NATURAL PHILOSOPHY. 


Pig. 66. 


the glass at n , a, b or c, will be equal to 1 lb. If 
the inside surface of the bottle be 100 square inches, 
then a pressure of 1 lb. upon the cork will produce 
a total force of 100 lbs., tending to burst the bottle. 

Illustrations of the transmission of pressure by 
Fig. 65. liquids. — Under 

, ^ n x - ■ i-ltp some circumstan- 
° ces this is more 

perfect than that by solids. Let a straight tube, A B, 
be filled with a cylinder of lead, and a piston, be 
fitted to the end of the tube. If now a force be ap¬ 
plied at O it will be 
transmitted with¬ 
out loss to P. If, 
instead, we use a 
bent tube, the force 
will be transmitted 
in the line of the arrow, and will act upon P but 
Fig - 67 - slightly, if at all. 

If, however, we 
fill the tube with 
water, the force 
will pass with¬ 
out diminution. 
With cords, pul¬ 
leys, levers, etc., 
we always lose 
about one-half 
of the force by 
friction; but this “ liquid rope” transmits it with 









HYDROSTATICS. 


105 


no sensible loss. Take a glass bulb and stem, as 
shown in Fig. 67, and fill it with water by the 
process explained under Thermometers. When 
full, if you are careful to let the stem slip loosely 
through the fingers as the bulb strikes, you may 
pound with it upon a smooth surface with all your 
strength. In this case, the force of the blow is 
instantly transmitted from the thin glass to the 
water, and'that being almost incompressible, makes 
the bulb nearly as solid as a ball of iron. 

If a Rupert’s drop be held in a vial of water, as in 
Fig. 68, and the 
tapering end bo 
broken, the force 
of the concussion 
will be transmit¬ 
ted to all parts of 
the glass and the 
vial will be in¬ 
stantly shattered. 

Water as a median - Fig. 69. 

ical power .—Take two 
cylinders, P and p, 
connected as in Fig. 

69, fitted with pistons 
and filled with water. 

Let the area of p be 
2 inches and that of P be 100 inches. Then, 
according to the principle of the equal pressure of 
liquids, a downward pressure of 1 lb. on each square 



Fig. 68. 

















! o6 NATJJBAL PHILOSOPHY. 

inch of the small piston will produce an upward 
pressure of 1 lb. on each square inch of the large 
piston. Hence a power of 2 lbs. would lift a weight 
of 100 lbs. This proportion may be increased by 
diminishing the size of p and increasing that of P, 

Fig. 70. 



so that the weight of a girl’s hand could lift a man- 
of-war. Water has been well termed the seventh 
mechanical power.” 

Hydrostatic Press. —Fig. 70 represents a press 
constructed on the principle just explained. As the 






























HYDROSTATICS. 


107 


piston a is forced down npon the water in the 
cylinder A by the workman, the pressure is trans¬ 
mitted through the bent tube of water d around 
under the large piston C which lifts up the platform 
K, and thus compresses the bales placed upon it. 
If the area of a is 1 inch and that of C 100 inches, 
then a force of 100 lbs. will lift 10,000 lbs. Still 
further to increase the efficiency of this press, the 
handle is a lever of the second class. If the distance 
of the hand from the pivot is ten times that of the 
piston, a P of 100 lbs. will produce a force of 1,000 
lbs. at a. This will become 100,000 lbs. at C. Hence, 
with a press of this size, a power of 100 lbs. will lift 
a weight or produce a pressure of 100,000 lbs. Ap¬ 
plying the principle of Mechanics, we see that here 
as elsewhere there is no force created, but that P x 
Vd = W X W d. The platform will ascend only 
TFoVinr P ar ^ of the distance the hand descends. This 
machine is used for baling hay and cotton for trans¬ 
portation ; for launching vessels; for testing the 
strength of ropes, chains, etc. The presses employed 
for raising the immense tubes of the Britannia 
Bridge were each capable of lifting 2,672 tons, or 
of throwing water in a vacuum to a height of nearly 
six miles. 



II. Liquids influenced by gkavity alone.— In this 
case there is no external pressure applied. The 
lower parts of a vessel of water must bear the 
weight of the upper parts. Thus each particle of 
water at rest is pressed downward by the weight 


io8 


NATURAL PHILOSOPHY 


of the minute column it sustains. It must, in turn, 
press in every direction with the same force, else it 
would be driven out of its place and the liquid would 
no longer be at rest. Indeed, when a liquid is dis¬ 
turbed in any manner it comes to rest; i. e., there is 
an equilibrium established only when there is this 
equality of pressure produced. In consequence of 
this constant pressure the following laws obtain: 

1st. Liquids at rest press downward , upward , and 
sidewise witJi the same force. —This may be illustrated 
by the following experiment. If the series of glass 

Fig. 71. 


12 3 4 



tubes shown in Fig. 71 be placed in a pail of water, 
the liquid will be forced up 1 by the upward pressure 
of the water, 2 by the downward pressure, 3 by the 
lateral pressure, and 4 by the three combined in dif¬ 
ferent portions of the tube. The water will rise in 
them all to the same height— i. e., to the level of the 
water in the pail. 

2d. The pressure increases until the depth. —The 
pressure at the depth of one foot is the weight of 
one cubic foot of water—viz., 62J lbs. (1,000 oz.); at 



















HYDROSTATICS. 


I09 


2 feet, twice tliat amount; and so on.* In sea-water 
it is greater, as that weighs 64.37 lbs. per cubic foot. 
At great depths this pressure becomes enormous. If 
a strong square glass bottle, empty and firmly corked, 
be sunk into the water, it will generally be crushed 
inward before it sinks ten fathoms. It is said that 
the Greenland whale sometimes descends to the 
depth of a mile, but always comes up exhausted and 
blowing blood. When a ship founders at sea, the 
great pressure forces the water into the pores of the 
wood, so that no part can ever rise again to the 
surface to reveal the fate of the lost vessel. 

3d. The pressure does not depend on the shape or size 
of the vessel .—In the apparatus shown in Fig. 72 the 
water rises to the 
same height in the 
variously shaped 
tubes, which com¬ 
municate with 
each other, what¬ 
ever may be their 
form or size. If 
more water be 
poured in one, it 
will rise higher in 
all the others. 



* Depth. Lbs. per sq. foot. 

1 ft. 62.5 

10 ft. 625. 

16 ft. 1,000. 


Depth. Lbs. per sq. foot. 

100 ft. 6,250 

1 mile, 830,000 

5 miles, 1,650,000 










I IO 


NATURAL PHILOSOPHY. 


Fig. 73. 





The Hydrostatic Bellows consists of two boards, 
each hinged on one side and resting on a rubber 

bag, to which is 
attached an up¬ 
right tube, A.— 
Water is poured 
in at A until the 
bag and tube are 
filled. The pres¬ 
sure of the column 
of water in the 
tube lifts the 
weights hung by 
crossbars beneath. 
Whether we use 
the tube A or B will make no difference in the weight 
supported, although the former holds ten times as 
much water as the latter. The tube C, however, 
being much longer, will exert a greater pressure. 
Fig. 74 . Another form of the same apparatus 
(Fig. 74) consists of two boards con¬ 
nected by a band of leather, in which 
a tall tube A is inserted. If this 
be filled with water, the pressure 
will be sufficient to lift a weight 
as much greater than the weight of 
the water in the tube as the area of 
the bellows-board is greater than the 
area of the tube. Applying again the 
principle of Mechanics, we see that if one ounce of 





























HYDROSTATICS. 


I I I 

water should raise a weight of 50 oz. one inch, then 
the water must fall 50 inches. 

A strong cask fitted with a 
small pipe 30 or 40 feet long, 
if filled with water will burst 
asunder. The pressure is as 
great as if the tube were of the 
same diameter as the cask. 

In a coffee or tea pot the small 
quantity of liquid in the spout 
balances the large quantity 
in the vessel. If it were not 
so, it'would rise in the spout 
and run out. 

The principle that a small 
quantity of water will thus 
balance another quantity, 
however large, or will lift any 
weight, however great, is fre¬ 
quently termed the “ Hydrostatic Paradox.” We see, 
however, that it is only an instance of the general law. 

4th. Water seeks its level.— This tendency is seen es¬ 
pecially in fountains and in the supply of water 
furnished to cities from an elevated reservoir. In 
Fig. 76 the tank is situated on a hill at the left, 
whence the water is conducted underground through 
a pipe to the fountain. The jet will rise, in theory , to 
the level of the surface, but in practice it falls short 
of this, owing to the friction at the nozzle of the pipe 
and in passing through the air, and the weight of the 


Fig. 75. 








NATURAL PHILOSOPHY. 


I I 2 



falling drops. It lias been thought that the Ro¬ 
mans knew nothing of this property of liquids, be¬ 
cause they built immense stone aqueducts a hundred 
miles in length, spanning valleys and rivers at vast 

Fig. 76. 


expense. Modern engineers simply carry the water 
in pipes through the valley or under the bed of the 
river, knowing that it will rise on the opposite side 
to its level. The ancients appear to have under¬ 
stood this principle, but could not make pipes capa- 
We of resisting the pressure. 

jirtesian wells are so named "because they have 
bfeen used for a long time in the province of Artois, 
in France. They were, however, employed by the 
Chinese from early ages for the purpose of procuring 
gas and salt water. 







HYDROSTATICS. 


1 ! 3 

Let A B and C D represent curved strata of clay 
impervious to water, and K K a layer of gravel and 
fine sand. The rain falling on the distant hills 
filters down to C D, and collects in this hollow 

Fig. 77. 



basin. If a well be bored at H, as soon as it reaches 
the stratum of gravel beneath, the water will rush 
upward, under the tremendous lateral pressure, to 
the height of the source, and often spout high in 
the air. The well at Grenelle, near Paris, is very 
celebrated. It is at the bottom of a great chalk- 
basin which extends many miles from the city. 
It is over 1,800 feet deep and furnishes 1,000,000 gal¬ 
lons daily. The wells of Chicago, on the level prai¬ 
rie, are about 700 feet deep, and discharge daily 
about 1,250,000 gallons of clear cold water. Tlio 
force with which the water comes to the surface in¬ 
dicates a head of 125 feet above Lake Michigan. 
Its source must be far away beyond Lake Superior, 




NATURAL PHILOSOPHY. 


I 14 

perhaps even beyond the Mississippi, toward the 
Rocky Mountains. Artesian wells are bored in the 
sands of Sahara; gardens are planted and dates 
flourish wherever water is supplied. Brigades of 
engineers are thus pushing forward the conquest of 
the African desert. 

Rules for the Calculation of Pressure. —1. To 
find the pressure on the bottom of a vessel. Multiply 
the area of the base by the perpendicular height, 
and that product by the weight of a cubic foot of the 
liquid.—2. To find the pressure on the side of a vessel. 
Multiply the area of the side by half of the perpen¬ 
dicular height, and that product by the weight of a 
cubic foot of the liquid. 

The pressure on the bottom of a cubical vessel 
full of water, is the weight of the water: on each 


Fig. 78. • 






HYDROSTATICS. 


11 s 

side, one-half; and on the four sides, twice the 
weight; therefore on the five sides, the pressure is 
three times the weight of the water. 

The Water-level.— The surface of standing water 
is said to be level— i. e ., horizontal to a plumb- 
line. This is true for small sheets of water, but for 
larger bodies an allowance must be made for the cir¬ 
cular figure of the earth. The curvature is 8 inches 
per mile ; 2 2 X 8 inches = 32 inches for two miles; 
3 2 X 8 inches = 72 inches for* three miles, etc. The 
spirit-level is an instrument used by builders for 

Fig. 7a 



levelling. It consists of a slightly curved glass 
tube so nearly full of alcohol that it holds only a 
bubble of air. When the level is horizontal, the bub¬ 
ble remains at the centre of the tube. 

Specific Gravity is the weight of a substance 
compared with the weight of the same bulk of an¬ 
other substance. It is really a method of finding the 
density of a body. Water is taken as the standard^ 

* A cubic inch of distilled water at a temperature of 62° F., 
with the barometer at 30 inches. This standard weighs 252.456 
grs.: 7,000 grs. make a pound Avoirdupois and 58,338 a gallon. 






! ! 5 NA TURAL PHIL OSOPHY. 

for solids and liquids, and air for gases. A cubic 
inch of zinc weighs seven times as much as a cubic 
inch of water; hence its specific gravity = 7. A 
cubic inch of carbonic acid gas weighs 1.52 times as 
much as the same volume of air; hence its specific 
gravity = 1.52. 

Buoyant Force of Liquids. —The cube abed is im¬ 
mersed in water. We see that the lateral pressure 
Fig. so. at a is equal to that at b, be¬ 

cause both sides are at the 
same depth; hence the body 
has no tendency toward 
either side of the jar. The 
upward pressure at c is 
greater than the downward 
pressure at d , because its 
depth is greater; hence the 
cube has a tendency to rise. 
This upward pressure is called the buoyant force of 
the water. Its law, discovered by Archimedes, is— 

The buoyant forge is equal to the weight of the liquid 
displaced. The downward pressure at d is the weight 
of a column of water whose area is that of the top 
of the cube, and whose perpendicular height is n d : 
the upward pressure at c is equal to the weight of a 
column of the same size whose perpendicular height 
is c n. The difference between the two, or the buoy¬ 
ant force, is the weight of a bulk of water equal to 
the size of the cube. 

The same is shown in what is called the “ cylinder 








HYDROSTATICS. 


17 


and bucket experiment.” The cylinder a exactly fits 
in the bucket b. The glass vessel in which the buck¬ 
et hangs is empty. The apparatus is balanced by 
weights placed in the scale-pan. Water is then 
poured into the glass vessel. Its buoyant force will 
raise the cylinder and depress the opposite scale-pan. 


Fig. 81. 



Let water be cautiously dropped into the bucket; 
when it is exactly full, the scales will balance again. 
This proves that a body in water is buoyed up by a 
force equal to the weight of the water it displaces. 

To find the specific gravity of a solid body by a hy - 




118 


NATURAL PHILOSOPHY. 


drostatic balance. —Weigh the body in air, and in wa¬ 
ter ; the difference is the weight of its bulk of water: 
divide its weight in air by its loss of weight in water ; 
the quotient is the specific gravity. Thus, sulphur 
loses one-half its weight when immersed in water; 
hence it is twice as heavy as water, and its specific 
gravity = 2. 

To find the specific gravity of a liquid by the specific- 
gravity flask. —This is a bottle which holds exactly 
1,000 grains of water. If it will hold 1,840 grains of 
sulphuric acid, the specific gravity of the acid is 1.84; 
if it will hold 13,500 grains of mercury, the specific 
gravity of that metal is 13.5. 

To find the specific gravity of a liquid by a hydrom¬ 
eter. —This instrument consists of a glass tube, closed 
at one end and having at the other a bulb contain¬ 
ing mercury or shot. A graduated scale is marked 
Fig. 82. upon the tube. The alcoholmeter 
is so balanced as to sink in pure 
water to the zero point at the bottom 
of the scale. As alcohol is lighter 
than water, the instrument will de¬ 
scend for every addition of spirits 
which is made. The degrees of the 
scale indicate the percentage of al¬ 
cohol. Instruments made in a sim¬ 
ilar manner are used for determining 
the strength of milk, acids, and solu¬ 
tions of various kinds. 

To find the weight of a given bulk of any substance .—- 













HTDROSTA TICS. 


!I 9 


Multiply the weight of one cubic foot of water by 
the specific gravity of the substance, and that pro¬ 
duct by the number of cubic feet. Ex.: What is 
the weight of three cubic feet of cork ? Solution : 
1,000 oz. x .240' :f == 240 oz.; 240 oz.x3 = 720 oz. 

To find the hulk of a given weight of any substance .— 
Multiply the weight of a cubic foot of water by the 
specific gravity of the substance, and divide the 
given weight by that product. The quotient is the re¬ 
quired bulk in cubic feet. Ex.: What is the bulk of 
20,000 oz. of lead ? Solution: 1,000 oz. x 11.36 * 
= 11,360; 20,000 -r- 11,360 == 1.76 + cu. ft 

To find the volume of a body. —Weigh it in water. 
The loss of weight is the weight of the displaced 
water. Then, as a cubic foot of water weighs 1,000 
oz., we can easily find the bulk of water displaced. 
Ex. : A body loses 10 oz. on being weighed in water. 
The displaced water weighs 10 oz. and is of a cu¬ 
bic foot; this is the exact volume of the body. 

Floating Bodies.— A very pretty experiment il- 


* Table of Specific Gravity. (See Rev. Chem., p. 288.) 


Iridium,. 21.80 

Platinum,. 21.63 

Gold,. 19.34 

Mercury,.. 13.5 

T.Pftrl 11.36 

Flint Glass,. 2.76 

Marble,. 2.70 

Quartz,. 2.65 

Chalk. 2.65 

Sulphur,. 2.00 

Liquids. 

Sulphuric Acid,.1.84 

Water from the Dead 

Sea,. 1.24 

Milk,. 1.03 

Silver,. 10.5 

Copper,. 8.9 

Tin,. 7.3 

Bone,. 1.99 

Phosphorus,. 1.83 

Sugar,. 1.60 

Sea-water,. 1.03 

Water,. 1. 

Absolute Alcohol,.79- 

Ether,.72 

Steel,. 7.81 

Coal,.1.30 

Iron,. 7.80 

Wax,.97 


Cast-iron,. 7.21 

Zinc,. 7. 

Ice,.93 

Potassium,.86 

Heavy Spar,. 4.43 

Diamond,. 3.50 

Pine Wood,.66 

Cork,.24 






































120 


NATURAL PHILOSOPHY. 


lustrative of this subject is represented in the cut. 
A glass jar is half full of water. An egg dropped 
in it sinks directly to the 
bottom. If, however, by 
means of a funnel with a 
long tube, we pour a little 
brine to the bottom be¬ 
neath the fresh water, the 
egg will gradually rise. We 
may vary the experiment 
by not dropping in the egg 
until we nave half filled 
the jar with the brine. 
The egg mil then fall to 
the centre, and there float 
like a balloon. Any solid 
substance dissolved in 
water simply fills the pores 
of the water without add¬ 
ing to its bulk. This in¬ 
creases its density and buoyant power. A person 
can therefore swim much more easily in salt than in 
fresh water. Bayard Taylor says that he could float 
on the surface of the Dead Sea, with a log of wood 
for a pillow, as comfortably as if lying on a spring 
mattress. Another traveller remarks, that on 
plunging in he was thrown out again like a cork; 
and that on emerging and drying himself, the crys¬ 
tals of salt which covered his body made him re¬ 
semble an “ animated stick of rock-candy.” 












IIYDROSTA TICS. 


121 


A piece of iron will float, if we hammer it into a 
vessel so that the weight of the water which it dis¬ 
places will exceed its own weight. An iron ship will 
not only float itself, but also carry a heavy cargo, 
because it displaces a great bulk of water. 

A body floating in water has its centre of gravity 
at the lowest point. Herschel tells an amusing 
story of a man who attempted to walk on water by 
means of bulky cork boots. Scarcely, however, had 
he ventured out ere the law of gravitation seized 
him, and all that could be seen was a pair of heels, 
whose movements manifested a great state of uneas¬ 
iness in the human appendage below. 

Fish are provided with an air-bladder, placed near 
the spine, by means of which they can rise or sink at 
pleasure. 

2^'actica? Questions.—1. Why do housekeepers test the strength of lye, 
by trying whether or not an egg will float on it? 2. How much water will 
it take to make a gallon of strong brine ? 3. Why can a fat man swim easier 
than a lean one ? 4. Why does the firing of a cannon sometimes bring to the 
surface the body of a drowned person ? Am. Because by the concussion it 
shakes the body loose from the mud or any object with which it is entangled. 

5. Why does the body of a drowned person generally come to the surface of 
the water, after a time ? Am. Because the gases which are generated by de¬ 
composition in the body render it lighter. 6. If we let bubbles of air pass up 
through a jar of water, why will they become larger as they ascend ? 7 . What Q gQlnJ', 
is the pressure on a lock gate 14 feet high and 10 feet wide, when the lock is ' —" 

full of water ? 8. Will a pail of water weigh any more with a live fish in it 

than without? 9. If the water filtering down through a rock should collect_ 

in a crevice an inch square and 250 feet high, opening at the bottom into a 
closed fissure having 20 square feet of surface, what would be the total pressure _ 
tending to burst the rock ? 10. Why can stones in water be moved so much 
more easily than on land ? 11. Why is it so difficult to wade in the water 
when there is any current ? 12. Why is a mill-dam or a canal embankment 
small at the top and large at the bottom ? 13. In digging canals and buildingf^y^) 
railroads, ought not the engineer to take into consideration the curvature of 
the earth? 14. Is the water at the bottom of the ocean denser than that atC Zl/*^ 
the surface? 15 . Why does the bubble of air in a spirit-level move as the 

6 



122 


NATURAL PHILOSOPHY. 


instrument is turned? 16 . Cannot a swimmer tread on pieces of glass and 
other sharp substances at the bottom of the water without harm ? 17. Will 
a vessel draw more water in fresh or in salt water? 18 . Will iron sink in mer¬ 
cury ? 19 . The water in the reservoir in New York is about fcO fe et above 
the fountain in the City Hall Park. What is the pressure upon a singfiHrrcfrof 
the pipe at the latter point ? 20 . Why does cream rise on milk ? 21 . If a ship 
founders at sea, to what depth will she descend ? (* It is a poetical thought 
that ships may thus sink into submarine currents and be carried hither and 
thither with their precious cargoes of freight and passengers, on voyages 
that know no end and toward harbors that they never reach.) 22 . There is a 
story told of a Chinese boy who accidentally dropped his ball into a deep hole 
where he could not reach it. He filled the hole with water, but the ball would not 
qui te float. He finally bethought himself of a lucky expedient, which was suc¬ 
cessful. Can you guess it? 23 . Which has the greater buoyant force, water or 
oil ? 24 . What is the weight of four cubic feet of cork ? 25 . How many ounces 
of iron will a cubic foot of cork float in water ? 26. What is the specific grav¬ 
ity of a body whose w r eight in air is 30 grs. and in water 20 grs. ? How much 
is it heavier than water ? 27. Which is heavier, a pail of fresh or one of salt 
water? 28 . The weights of a piece of syenite-rock in air and water were^ 
3941.8 grs. and 2607.5 grs. Find its specific gravity. 29 . A specimen of 
green sapphire from Siam weighed in air 21.45 grs. and in water 16.33 grs.; re 
quired its specific gravity. 30 . A specimen of granite weighs in air 534.8 grs 
and in water 334.6 grs.; what is its specific gravity ? 31. What is the bulk o 
a ton of iron ? A ton of gold ? A ton of copper ? 32. What is the weight of a- 
cube of gold 4 feet on each side ? 3 3 . A cistern is 12 feet long, 6 feet wide, and 
10 feet deep; when full of water, what is the pressure on each side ? 34 . Why 
does a dead fish always float on its back ? 35. A given bulk of water weighs 
62.5 grs., and the same bulk of muriatic acid 75 grs. What is the specific grav¬ 
ity of the acid ? Ans. 1.2. 36. A vessel holds 10 lbs. of water; how muc 
mercury would it con^n ? 37. A stone weighs 70 lbs. in air and 50 in wa- 
ter ; what is its bulk ^8- A follow ball of iron weighs 10 lbs.; what must 
be its bulk to float in &a&r ? - 


PM 


HYDRAULICS. 

Hydraulics treats of liquids in motion. In this, 
as in Hydrostatics, water is taken as the type. In 
theory, its principles are those of falling bodies, but 
they are so modified by various causes, that in prac¬ 
tice they cannot be relied upon except as verified by 
experiment. The discrepancy arises from changes 



HYDRAULICS. 


123 


of temperature which vary the fluidity of the liquid, 
from friction, the shape of the orifice, &c. 

The velocity of a jet is the same as that of a body 
falling from the surface of the ivater. —We can see that 
this must be so, if we recall two principles we have 
already learned. First, that “ a jet will rise to the 
level of its sourceand second, that “ to elevate a 
body to any height, it must have the same velocity 
that it would acquire in falling that distance.” It 
follows, therefore, that the velocity of a jet depends 
entirely on the height of the liquid^above the orifice, 
and that all liquids will issue with the same velocity 
at the same depth. Molasses ought to flow with the 
same speed as mercury, for the same reason that a 
guinea falls in the same time as a feather. The ap¬ 
plication of this principle is of course modified by 
the temperature, and varic us other causes. 

To find the velocity of a jet of water. —We use here 
the 4th equation of falling bodies, v = 2 -fgd, in 
which d is the distance of the orifice below the sur¬ 
face of the water. Ex. : The depth of water above 
the orifice is 64 feet; required the velocity. Substi¬ 
tuting 64 for c?, we have v = 2 \/l6 x 64 = 64 feet. 

To find the quantity of ivater discharged in a given 
time. —Multiply the area of the orifice by the veloci¬ 
ty of the water, and that product by the number of 
seconds. Ex.: What quantity of water will be dis¬ 
charged in five seconds from an orifice having an area^ 
of J a square foot, at a depth of 16 feet ? At that 
depth, v = 2 V 16 x 16 = 32 feet per second ; multi- 




124 


NATURAL PHILOSOPHY. 


plying by we have 16 cubic feet as the amount dis¬ 
charged in one second and 80 cubic feet in five sec¬ 
onds. In practice, however, it is found that but 62 
per cent, of this amount can be realized. 

Effect of Tubes. —If we examine a jet of water, we 
shall see, just outside the orifice, its size is decreased 
to about | that at the opening. This is caused by the 
water producing cross currents as it flows from dif¬ 
ferent directions toward the orifice. If a tube of a 
length twice or thrice the diameter of the opening be 
inserted, the water adheres to the sides of the tube, 
so that there is no contraction, and the flow is in¬ 
creased to 82 per cent, of the theoretical amount. 

If the tube be conical, and inserted with the large 
end in the opening, the discharge may be increased 
to 92 per cent.; and strangely enough, by inserting it 
with the smaller end next the orifice, the amount 
exceeds that indicated by theory as much as 25 per 
cent. It seems in this case to be made so easy for 
the water to run, that more is coaxed out than ought 
to go. Long tubes or curves, however, by their 
friction, largely diminish the flow of water. It is 
said that a single right-angle will decrease it one- 
half, while an inch pipe 200 feet long will discharge 
only J as much water as one an inch long. 

Flow of Water in Eivers. —A fall of only three 
inches per mile is sufficient to give motion to water, 
and produce a velocity of as many miles per hour. 
The Ganges descends but 800 feet in 1,800 miles. Its 
waters require a month to move down this long 


HYDRAULICS. 


125 


inclined plane. A fall of 3 feet per mile will make 
a mountain torrent. The current moves more swiftly 
at the centre than near the shores or bottom of a 
channel, since there is less friction. 

W ater-wheels are machines for using the force 
of falling water. By means of bands or cog-wheels 
the motion of the wheel is conducted from the axle 
into the mill. The principle is that of a lever with 
the P acting on the short arm. In this way, the 
movement of the slow creaking axle reappears in the 
swiftly buzzing saw or flying 
spindle. Water-wheels are of 
four classes— The Overshot , Un¬ 
dershot , Breast , and Turbine 
wheels. The Over shot-wheel has 
on its circumference a series of 
buckets which receive the water 
as it flows out of a sluice , C. The 
buckets are so made as to hold the water as they 
descend on one side, and to empty it as they come 
up on the other. Overshot-wheels are valuable 
where a great fall can be secured, since they require 
but little water. They are made of great size. One 
at Cohoes, N. Y., is 96 feet high. If P denotes the 
weight of the water and d the distance it falls, then 
the total force = P d. Of this amount 80 per cent, 
can be secured in the best wheels of this and the 
third class. 

The Undershot-wheel , instead of buckets has merely 
projecting boards or floats , which receive the force 





26 


NATURAL PHILOSOPHY. 


of the current. It is of use where there is little fall 
and a large quantity of water. It is said to utilize 
only 20 per cent, of the force of the water. 


Fig. 85. 




Fig. 86. 


The Breast-ivlieel is a medium between the two 
before named, as may be seen in Fig. 86. 

The Turbine-wheel differs essentially from the 
others named. It is placed horizontally, and is en¬ 
tirely immersed in the water. In the figure, G is 
the dam and D A the spout by which the water is 
furnished to the wheel. E is a scroll-like casing en¬ 
circling the wheel, and open at the centre above and 
below. The axis of the wheel is the cylinder f 9 from 
which radiate plane-floats against which the water 
strikes. To confine the water at the top and the bot¬ 
tom is a circular disk attached to the cylinder and the 
floats. In these disks are the swells for discharging the 
water. They project above and below, as seen in the 
figure. They commence near the cylinder, and swell¬ 
ing outward scroll-shaped, form openings curved 
toward the cylinder, thus emptying the water in a 
direction opposite to that in which it enters the 





HYDRAULICS. 


I27 


wheel. This form utilizes as high as 90 per cent, of 
the force. F is a band-wheel which conducts the power 
to the machinery. The principle of the turbine is 

Fig. 87. 


F 



that of the unbalanced pressure of a column of wa¬ 
ter. It is finely illustrated in the old-fashioned 
Barker’s Mill or Reaction Wheel. This consists of 
an upright cylinder with horizontal arms, on the 
opposite sides of which are small apertures. It rests 
in a socket, so as to revolve freely. Water is sup¬ 
plied from a tank above. If the openings in the 
arms are closed, when the cylinder is filled with 
water the pressure will be equal in all directions and 




























128 NATURAL PHILOSOPHY . 


tlie machine will be at rest. If now we open an 
aperture* the pressure is relieved on that side, and 

the arm flies back¬ 
ward with the un¬ 
balanced pressure of 
the column of wate*- 

above. - 

Waves are pro¬ 
duced by the friction 
of the wind against 
the surface of the 
water. A light wind 
forms merely ripples; 
these increase out in 
the open sea, as 
wave is raised upon 
wave, until they be¬ 
come great billows 
which constantly 
surge to and fro, so 
that the sea is never 
at rest. The wind 
raises the particles of 
water and gravity 
draws them back again. They thus vibrate up and 
down, but do not advance. The forward movement 
of the wave is only an illusion. The form of the 
wave progresses, but not the water of which it is 
composed, any more than the thread of a screw 
which we turn in our hand; or the undulations of a 















HYDRA ULIL'S. 


I29 



rope or carpet which is being shaken; or the stalks 
of grain which bend in billows as the wind sweeps 
over them. If we watch a buoy in the harbor or 
a body floating on the surface of water, we shall 
see that it moves forward on the crest of each 
wave through a few feet or inches, according to 
the length of the wave; then stops, moves back¬ 
ward in the hollow; stops, and again moves forward 
as before on the crest of the next wave. The mole¬ 
cules of water vi¬ 
brate to and fro in 
an elliptical path. 

Thus, let the figure 
represent two suc¬ 
cessive wave-crests and the hollow between. While 
the whole wave moves from the position A B to that 
of C D, the molecules of water only move backward 
or forward through a distance A B or CD; forward 
on the crest of the wave and backward in the hollow 
as shown by the arrows. The velocity of the particles 
may be much slower than that of the progressive 
motion of the wave. It is said that in an earthquake, 
the velocity of the particles of the shaken ground is 
often only three feet per second, while the earth-wave 
moves across the country at the rate of 3,000 feet per 
second. 

Near the shore the character of waves is some¬ 
what changed. The oscillations are shorter, and as 
the waves do not balance those in the deeper water, 
they are forced forward till the lower part of each wave 
6 * 






i3° 


NATURAL PHILOSOPHY. 


is checked by the friction on the sandy beach, and 
the upper part curls over and falls beyond. The size 
of “ mountain billows” has been much exaggerated. 
The ocean is probably undisturbed below the depth 
of 30 feet. The highest wave, from the deepest 
“trough” to the very topmost “crest,” is only 43 feet. 
The corresponding parts of different waves are 
termed like phases . The distance between two like 
phases, or between the crests of two succeeding 
waves, is called a ivave-lengtli. Opposite phases are 
those parts which are vibrating in different direc¬ 
tions, as the point midway in the front of one wave 
and another midway in the rear of the next wave. 

A tide-wave maybe setting steadily toward the 
west; waves from distant storms may be moving upon 
this; and above all, ripples from the breeze then blow¬ 
ing may diversify the surface. These different systems 
will each be entirely distinct, yet the joint effect may 
be very peculiar. If any two systems exactly coin¬ 
cide with like phases ,—the crest of one meeting the 
crest of the other, and the furrow of one meeting the 
furrow of the other,—the resulting wave will have a 
height equal to the sum of the tiuo. If any two coin¬ 
cide with opposite phases,—the hollow of one strik¬ 
ing the crest of another,—the height will be the dif¬ 
ference of the two . Thus, if in two systems having the 
same wave-length and height, one is exactly half 
a length behind the other, they will mutually destroy 
each other. This is termed the interference of waves. 
The manner in which different waves move among 


HYDRAULICS. 


13 1 



and upon each other, is seen by dropping a handful 
of stones in water and watching the waves as they 

6 


rte. 90. 




circle out from the various centres in ever-widening 
. curves. In the figure is shown the beautiful appear¬ 
ance these waves present when reflected from the 
sides of a vessel. 


The application of these principles in Sound and 
Light will be found very important. 

^Practical Questions.— 1. How much more water can be drawn from 
faucet 8 feet than from one 4 feet below the surface of the water in a cistern ? 
2. How much water will be discharged per second from a short pipe hav¬ 
ing a diameter of 4 inches and a depth of 48 feet below th'e surface of the wa- 
ter ? 3. When we pour molasses from a jug, why is the stream so much 
larger near the nozzle than at some distance from it? 4 . Ought a faucet to 
extend into a barrel beyond the staves ? 5. What would be the effect if both 
openings in one of the arms of Barker’s Mill were on the same side ? 




/ 






















1 3 2 


NATURAL PHILOSOPHY. 


PNEUMATICS. 

Pneumatics treats of the general properties and the 
pressure of gases. Since the molecules move among 
each other more freely even than those of liquids, the 
conclusions at which we have arrived with regard to 
transmission of 'pressure , buoyancy and specific gravity 
apply also to gases. Its principles obtain in all gas¬ 
eous bodies, but as air is the most abundant gas, it is 
taken as the type of the class, as water is of liquids. 

The Air-pump is shown in its essential features 
in Fig. 91. A is a glass receiver standing on an oiled 
pump-plate. The tube D, connecting the receiver 
with the cylinder, is closed by the valve E opening 
upward. There is a second valve, P, in the piston, 
also opening upward. Suppose the piston is at the 
bottom and both valves 
shut. Let it now be raised, 
and there will be a vacuum 
produced in the cylinder; 
the expansive force of the 
atmosphere in the receiver 
will open the valve E and 
drive the air through to fill 
this empty space. When the piston descends, the 
valve E will close, while the valve P will open, 
and the air will pass up above the piston. On ele¬ 
vating the piston a second time, this air is removed 













PNETJMA TICS. 


*33 


Fig. 92. 



The Air-pump. 


from the cylinder, while the air from the receiver 
passes through as before. At each stroke a portion 
of the atmosphere is drawn off; but 
the expansive force becomes less and 
less, until finally it is not sufficient 
even to lift the delicate valves. For 
this reason a perfect vacuum cannot 
be obtained. 

Properties of the Air.— Weight — 

Exhaust the air from a flask which 
holds 100 cubic inches, and then bal¬ 
ance it accurately. If now we turn 
the stop-cock, the air will rush in with 


























134 


NATURAL PHILOSOPHY. 


a whizzing noise and the flask will descend. We shall 
have to add about 31 grains to restore the equipoise. 

Elasticity and compressibility .—These properties are 
shown in the common pop-gun. We compress the 
atmosphere in the barrel until the elastic force be¬ 
comes so great as to drive out the stopper with a 
loud report. As we crowd down the piston we feel 
the elasticity of the air yielding to our strength, like 
a cushion or a bent spring. 


Fig. 94. 



The bottle-imps , or Cartesian divers, illustrate the 
same properties. Fig. 94 represents a very simple 


















PNEUMA TICS. 


135 


form of this experiment. The cover of a common 
fruit-jar is fitted with a small tin tube, which is in¬ 
serted into a syringe-bulb. The jar is filled with 
water and the divers placed within. These are hol¬ 
low images of glass, having each a small opening at 
the end of the curved tail. If we squeeze the bulb, 
the air will be forced into the jar and the water will 
transmit the pressure to the air in the image. This 
being compressed, the water will enter, and the spe¬ 
cific gravity being increased, the diver will descend. 
On relaxing the grasp of the hand on the bulb, the 
air will return into it, the air in the image will ex¬ 
pand by its elastic force driving out the water, and 
the diver, thus lightened of his ballast, will ascend. 
The nearer the image is to the bottom, the less force 
will be required to move it. With a little care it can 
be made to respond to the slightest pressure, and 
will rise and fall as if instinct with life. This experi¬ 
ment shows also the buoyant force of liquids, their 
transmission of pressure in every direction, the in¬ 
crease of the pressure in proportion 
to the depth, and the principle of 
Barker’s Mill. 

Expansibility .—Let a well-dried 
bladder be partly filled with air 
and tightly closed. Now place it 
under the receiver and exhaust the 
air. The air within the bladder ex¬ 
panding will swell and oftentimes burst it into shreds. 

Take two bottles partly filled with colored water. 


Fig. 95. 







NATURAL PHILOSOPHY. 


136 

Let a bent tube be inserted tightly in A and 
loosely in B. Place this apparatus under the re¬ 
ceiver and exhaust the air. The expansive force of 
the air in A will drive the water 
over into B. On readmitting the 
air into the receiver, the pressure 
will return the water into A. It 
may thus be driven from one 
bottle to the other at pleasure. 
Pressure of the Air.— If we place the hand¬ 
glass on the plate of the Air-pump, covering it with 
one hand, on exhausting the air 
we shall soon find the pressure to 
become painful. Tie over one 
end of the glass a piece of well- 
soaked bladder. When thor¬ 
oughly dry, exhaust the air from 
it as before, and the membrane 
will burst with a sharp report. 

The Magdeburg Hemispheres 
are named from the city in which 
Otto Guericke, their inventor, resided. They con¬ 
sist of two small brass hemispheres, which fit closely 
Fig. 98. together, but may be 

separated at pleas¬ 
ure. If, however, the 
air be exhausted 
from within, the 
strength of several 
persons will be required to pull them apart. No 



Fig. 97. 



Fig. 96. 






PNEUMATICS . 


*37 


Fig. 99. 



matter in wliat position the hemispheres are held, 
we shall find the pressure the same. 

Upivard Pressure of the Air . 

—Fill a tumbler with water, and 
then lay a sheet of paper over 
the top. Quickly invert the 
glass, and the water will be sup¬ 
ported by the upward pressure of 
the air. 

Within the glass cylinder 
shown in Fig. 100 there is a piston working air¬ 
tight. Connect C with the pump by means of a 
rubber tube and exhaust the 
air. The weight will leap 
jrfp as if caught by a spring. 

\ Buoyant Force of the Air . 

—The principle of Archi¬ 
medes holds true in gases as 
in liquids. Illustrations of 
this abound in common life. 

Smoke and other light sub¬ 
stances float in the air, as 
wood does in water, because 
they are lighter and are 
buoyed with a force equal to 
the weight of the air they displace. In Fig. 101 
we have a hollow sphere of copper, which is ex¬ 
actly balanced in the air by a solid lead weight, 
but instantly falls on being placed under the re¬ 
ceiver and the air exhausted. This shows that its 









NATURAL PHILOSOPHY. 


138 

weight was partly sustained by the buoyant force of 
the air. 

The pressure of the air sustains a column of mercury 


Fig. 101. Fig. 102. 



30 inches high , of water 34 
feet high , and is 15 lbs. per 
square inch. 

Take a strong glass 
tube about three feet in 
length, and tie over one 
end a piece of well-soaked 
bladder. When thorough¬ 
ly dry, fill the tube with 
mercury, and invert it in a cup of the same liquid. 
The mercury will sink to a height of about 30 inches. 
If the area of the tube be one inch, this amount of 
the metal will weigh about 15 lbs. The weight of 
the column of mercury is equal to the downward 
pressure on each square inch of the surface of the 












PNEUMATICS. 


139 

mercury in the cup. Hence we conclude that the 
pressure of the atmosphere is 15 lbs. per square 
inch, and will balance a column of mercury 30 
inches high. As water is 13^ times lighter than 
mercury, it is evident that the same pressure would 
balance a column of that liquid 13J times higher, 
or 33J feet. On account of the unwieldy length 
of the tube required to exhibit the column of 
water, it is not easy to verify this last statement. 
It may, however, be prettily illustrated in the fol¬ 
lowing manner. Pour on the mercury in the cup 
(Fig. 102) a little water colored with red ink. Now 
raise the end of the tube carefully above the surface 
of the metal, but not above that of the water which 
will immediately rise in the tube, the mercury passing 
down in beautifully beaded globules. The mercu¬ 
rial column was only 30 inches high, while the water 
will entirely fill the tube. Finish the experiment by 
puncturing the bladder with a pin, when the water 
will instantly fall to the cup below. 

The pressure of the air varies .—We live on the bed 
of an aerial ocean whose invisible tides surge around 
us on every side. More restless than the sea, its 
waves beat to and fro, stirred by a multitude of causes. 
Changes of temperature, moisture, &c., constantly 
vary the weight of the air, and consequently change 
the height of the column of liquid which it can sup¬ 
port. There is also a diurnal variation, due to the 
heat of the sun,—slight indeed, yet so marked that 
Humboldt says that the play of the mercurial column 


140 


NATURAL PHILOSOPHY. 


could be used to indicate the hour of the day. The 
pressure of the air increases with the depth. Hence, 
in a valley its weight is greater than on a mountain. 

Fig. 103. Fig. 104. 



The figures given in the last paragraph apply only 
to the level of the sea and the temperature of 60° F. 
They are an average of all the variations, and are 
considered the standard for reference. 




























PNEUMATICS. 


14 1 

Mariotte's Law. —Fig. 103 represents a long bent 
glass tube with the end of the short arm closed. 
Four mercury into the long arm until it rises to the 
point marked zero. It stands at the same height in 
both arms, and there is an equilibrium. The air 
presses on the mercury in the long arm with a force 
equal to a column of mercury 30 inches Fi g . 105 . 
high, and the elastic force of the air confined 
in the short arm is equal to the same amount. 

Let us now pour additional mercury into the 
long arm until it stands at 30 inches. (Fig. 

104.) We have evidently doubled the pres¬ 
sure. If we look at the short arm, we shall 
find that the air is condensed to one-half 
its former dimensions, and of course the 
expansive force must be doubled. We there¬ 
fore conclude that the elasticity of a gas in¬ 
creases and the volume diminishes in proportion 
to the pressure upon it. 

The Barometer is an instrument for 
measuring the pressure of the air. It con¬ 
sists essentially of the tube and cup of mer- * 
cury shown in Fig. 102. A scale is attached 
for convenience of reference. The barom¬ 
eter is used ( 1 ) to indicate the weather, and 
( 2 ) to measure the height of mountains. 

It does not absolutely foretell the charac¬ 
ter of the weather. It simply shows the 
varying weight of the air, from which we must draw 
our own conclusions. A continued rise of the mer- 











142 


NATURAL PHILOSOPHY. 


cury indicates fair weather, and a continued fall, foul 
weather. 

Since the pressure diminishes as one ascends above 
the level of the sea, the observer ascertains the fall 
of the mercury in the barometer, and the tempera¬ 
ture by the attached thermometer ; and then, by ref¬ 
erence to carefully prepared tables, easily determines 
the height. 

' Water-barometer .—Mercury is used for filling the 
^barometer because of its weight and its low freezing- 
point. Water would require a tube about 34 feet in 
length. It is said that the first barometer was filled 
with that liquid. The inventor, Otto Guericke, a 
wealthy burgomaster of Magdeburg, Saxony, erected 
a tall tube reaching from a cistern in the cellar up 
through the roof of his house. A tall wooden image 
—life-size—was placed within the tube, floating upon 
the water. On fine days, this novel weather-prophet 
would rise above the roof-top and peep out upon the 
queer old gables of that ancient city, while in foul 
weather he would retire to the protection of the gar¬ 
ret. The accuracy of these movements attracted the 
attention of the neighbors. Finally, in their inno- 
cency, becoming suspicious of Otto Guericke’s piety, 
they openly accused him of being in league with the 
devil. So the offending philosopher relieved this 
wicked wooden man from longer dancing attendance 
upon the weather, and the staid old city was once 
more at peace. ^3 4 

Pumps.— Two varieties are in common use. These 
are the Lifting and the Forcing pump. 


PNEUMATICS. 



143 

The Lifting-pump contains two valves opening 
upward—one, a, at the top of the suction-pripe, B; 
the other, c, in the piston. Suppose the handle to 
be raised, the piston at the bottom of the cylinder 

Fig. 106. 


and both valves closed. Now depress the pump- 
handle and elevate the piston. This will produce a 
partial vacuum in the suction-pipe. The pressure of 
the air on the surface of the water below will force 
the water up the pipe, open the valve, and fill the 
chamber, as seen in the first figure. Let the pump- 
handle be elevated again, and the piston depressed. 
The valve a will now close, the valve c will open and 

























144 


NATURAL PHILOSOPHY. 


the water will flow through it above the piston, as in 
the second figure. When the pump-handle is low¬ 
ered the second time and the piston elevated, the wa¬ 
ter is lifted up to the spout, whence it flows out; 
while at the same time the lower valve opens and 
the water is forced up from below by the pressure 
of the air, as in the third figure. 

If the valves and piston were fitted air-tight, the 
water could be raised 34 feet (more exactly 13 J times 
the height of the barometric column) to the lower 
valve, but owing to various imperfections it com¬ 
monly reaches only 28 feet. 

Fig. 107. J J 

± or a similar reason we some¬ 
times find a dozen strokes neces¬ 
sary to “ bring water.” 

The Force-pumjp has no valve 
in the piston. The water rises 
above the lower valve as in the 
lifting-pump. When the piston 
descends, the pressure opens 
the valve O and forces the wa¬ 
ter up the pipe D. This pipe 
may be made of any length, 
and thus the water driven to 
any height. 

The Fire-engine consists of 
two force-pumps with an air- 
chamber. The water is driven 
by the pistons m, n, alternately 
into the chamber R, whence the air, by its ex- 














PNEUMATICS. 


145 



pansive force, throws it out in a. continuous stream 
through the hose-pipe attached at Z. 

Fig. 108. 


The Siphon consists simply of a tube bent in the 
form of the letter U, having one arm longer than the 
other. We insert the short arm in the water, and 
then applying the mouth to the other, exhaust the 
air. The water will immediately begin to flow from 
the long arm, and continue until the lower end of the 
short arm is uncovered, or until the water in the two 
vessels comes to the same level. A very instructive 
variation of this experiment may be given if we color 
the water with red ink, and then allow it to run from 
one tumbler into another until just before the flow 



















146 


NATURAL PHILOSOPHY. 


would cease; then quickly elevate the vessel con¬ 
taining the long arm, carefully keeping both ends of 
the siphon under the water, when the flow will set 
back to the first vessel. Thus we may alternate, 


Fig. 109. 



backward and forward, until we see clearly that the 
water flows always to the lower level from the long 
arm, and ceases whenever the water in the two vessels 
reaches the same level. 

The Theory of the Siphon .—The pressure of the air 
at h holds up the column of water a h, and the up¬ 
ward pressure is the weight of the air less the weight 
the column of water a h. The upward pressure 
at d is the weight of the air minus the weight of the 
column of water c d . Now c d is less than a h, 

















PNEUMATICS. 


14 7 


Fig. 110. 


therefore the pressure at d is greater than that at b, 
and the water in the tube is driven toward the longer 
arm by a force equal to the difference between th 
two arms. 

The Pneumatic Inkstand is filled by pouring in 
the ink when the bottle is tipped so that the nozzle 
is at the top. The pressure 
of the air will then hold the 
ink in the stand. When it 
is used below the level of o, 
a bubble of air passes in, 
forcing the ink into the noz¬ 
zle as desired. 

The Ancients noticing 
how the air rushes in to 
fill any empty space, ex¬ 
plained the fact by saying, 

“ Nature abhors a vacuum.” This principle an¬ 
swered the purpose of philosophers even in modern 
times. In the 17th century, when workmen were 
employed by the Duke of Tuscany to dig a very 
deep well near Florence, they found to their sur¬ 
prise that the water would not rise in the pump as 
high as the lower valve. They applied in their 
dilemma to Galileo. The old philosopher replied—- 
half in jest we hope, certainly he was half in earnest— 
“ Nature does not abhor a vacuum beyond 34 feet.” 

Three opposing forces act upon the Air —viz: 
Gravity, which binds it to the earth, and the Centrif¬ 
ugal and the Repellant [heat] forces, which tend to 





148 


NATURAL PHILOSOPHY. 


hurl it off into space. Under tlie action of the latter 
forces the atmosphere, like a great bent spring, is 
ready to bound away at the first opportunity; but the 
attraction of the earth holds it firmly in its place. 

Height and Density of the Air.—F ifty miles 
has been taken as the extreme limit of the atmos¬ 
phere. The latest investigations, however, indicate 
that there is an extremely rarefied air at the height 
of perhaps 500 miles. Its density rapidly dimin¬ 
ishes as we ascend. At the height of 3J miles it is 
but one-half that at the sea-level. At 40 miles the 
atmosphere is rare as in the vacuum of an air-pump. 


530^ 


Radical Questions .—1. Why must we make two openings in a ban 
of cider when we tap it W hat is the weight of 10 cubic feet of air 
What is the pressure of the air on 1 square rod of land ? 4. What is the pres-^ 
sure on a pair of Magdeburg hemispheres 4 inches in diameter? 5. How 
high a column of water can the air sustain when the barometric column stands 
at 28 inches? 6. If we should add a pressure of two atmospheres (30 lbs. to-' “ 
the square inch), what would be the volume of 100 cubic inches of common v ^ xj 
air ? 7. If, while the water is running through the siphon, we quickly lift the 
long arm, what is the effect on the water in the siphon ? If we lift the entire 
siphon? 8. When the mercury stands at 29£ inches in the barometer, how 
high above the surface of the water can we place the lower pump-valve ? 9 . 

Why cannot we raise Avater by means of a siphon to a higher level? 10.**— 

If the air in the chamber of a fire-engine be condensed to Vie its former bulk, 
what will be the pressure due to the expansive force of the air on every square 
inch of the air-chamber ? 11. What causes the bubbles to rise to the surface 
when we put a lump of loaf-sugar in hot tea ? 12. To what height can a bal- 

loon ascend ? What Aveight can it lift ? 13. The rise and fall of the barometric 
column shows that the air is lighter in foul and heavier in fair weather. Why 
is this ? Ans. In fair Aveather the moisture of the air is an invisible vapor 
mingled with it and adding to its pressure, while in foul Aveather the vapor is 
separated in the form of clouds. 14. When smoke ascends in a straight lino _ 
from chimneys, is it a proof of the rarity or density of the air ? 15. Why do 

\ve not feel the heavy pressure of the air upon our bodies ? 1G. Is a bottle 
empty Avhen filled with air ? 17. Why is it so tiresome to Avalk in miry clay ? 

Ans. Because the upAvard pressure of the air is removed from our feet. 18. 

How does the variation in the pressure of the air affect those who ascend 
lofty mountains ? Who descend in diving-bells ? 19. Explain the theory of 

“ sucking cider” through a straw. 



“ Science ought to teach us to see the invisible as well as the visible in 
nature: to picture to our mind’s eye those operations that entirely elude the 
eye of the body ; to look at the very atoms of matter, in motion and in rest, 
and to follow them forth into the world of the senses.”— Tyndall,. 



































ACOUSTICS. 

Acoustics treats of sound.* 

Sound is produced by vibrations. —By lightly 
tapping a receiver or even a glass fruit-dish, you can 
see that the sides are thrown into motion. Fill a 
goblet half full of water, and wetting your finger, rub 
it lightly around the upper edge of the glass. The 
sides will vibrate, and tiny waves corresponding to 
these movements will ripple the surface of the water. 
The vibrations of a tuning-fork are very distinct. 
Hold a card close to its prongs, and you can hear 

* The term sound is used in two senses—the subjective (that which has ref¬ 
erence to our mind) and the objective (that which refers only to the objects 
around us). (1) Sound is the sensation produced upon the organ of hearing 
by vibrations in matter. In this use of the word there can be no sound where 
there is no ear to catch the vibrations. An oak falls in the forest, and if there 
is no ear to hear it there is no noise, and the old tree drops quietly to its 
resting-place. Niagara’s flood poured over its rocky precipice for ages, but 
fell silently to the ground. There were the vibrations of earth and air, but 
there was no ear to receive them and translate them into sound. When, 
however, the first foot trod those primeval solitudes, and the ear first felt the 
pulsations from the torrent, then first the roaring cataract found a voice 
and broke its lasting silence. A trumpet does not sound. It only carves 
the air into waves. The tympanum is the beach on which these break into 
sound. (2) Sound is those vibrations of matter capable of producing a 
sensation upon the organ of hearing. In this use o? the word there can be a 
sound in the absence of the ear. An object falls and the vibrations are pro¬ 
duced, though there may be no organ of hearing to receive an impression from 
them. 




152 


NATURAL PHILOSOPHY. 


the repeated taps. Place jour cheek near them, and 
you will feel the little puffs of wind. Insert the 
handle between your teeth, and you will experience 
the indescribable thrill of the swinging metal. The 
tuning-fork may be made even to draw the outline 
of its vibrations upon a smoked-glass. Fasten upon 
one prong a sharp-pointed piece of metal, and on 


Fig. 111. 



drawing the fork along as in the figure, a sinuous 
line will show the width (amplitude) of the vibrations. 

From many similar experiments it is believed that 
when a body is struck its molecules are thrown into 
motion, and that all sound is produced by vibrations. 

Hoio sound is conveyed through the air .—Let us im¬ 
agine the prong of the tuning-fork used in the last 
experiment to advance, condensing the air in front 
of it, and then to recede, leaving behind it a partial 
vacuum. This process is repeated until the fork 
comes to rest, and the sound ceases. Each vibration 
of the prong produces a sound-wave of air, which 
contains one condensation and one rarefaction. In 
water, we measure a wave-length from crest to crest; 
in air, from condensation to condensation. The con¬ 
densation of the sound-wave corresponds to the crest 







ACOUSTICS. 


53 


of the water-wave, and the rarefaction of the sound¬ 
wave to the hollow of the water-wave. In the figure, 


Fig. 112. 

A a cl' b V c o' d A’ 



the dark spaces a, b, c, d represent the condensations, 
and a , b\ c the rarefactions; the wave-lengths are 
the distances ab , be , cd. 

If we fire a gun, the gases which are produced ex¬ 
pand suddenly and force the air outward in every 
direction. This hollow shell of air thus condensed 
imparts its motion to that next to it, while it springs 
back by its elasticity and becomes rarefied. The 
second shell rushes forward with the motion re¬ 
ceived, then bounds back and becomes rarefied. 
Thus each shell of air takes up the motion and 
imparts it to the next. The wave, consisting of 
a condensation and a rarefaction, proceeds onward. 
It is, however, as in water-waves, a movement of the 
form only, while the particles vibrate but a short 
distance to and fro. The molecules in water-waves 
oscillate vertically ; those in sound-waves horizontally , 
or parallel to the line of motion. 

7 * 


























154 


NATURAL PHILOSOPHY. 


If a bell be rung, the air adjacent to it is set in 
motion: thence, by a series of condensations and 
rarefactions, the vibrations of the bell are conveyed 
to the ear, and thus produce the sensation of sound. 


Fig. 113. 



When we speak, we do not shoot the air which we 
expel from our lungs into the ear of the listener. We 
simply condense the air just before our mouth and 
throw it into vibrations. Thus a sound-wave is 
formed. This travels onward and spreads in every 
direction in the form of a sphere of which we are the 
centre. 

Sound will not pass out of a vacuum.— In 
the figure, B, the bell, is struck by clock-work which 
may be set in motion by the sliding-rod r. The ap¬ 
paratus is suspended by means of silk cords, that no 
vibration may be conducted through the pump itself. 
If the air be exhausted, the sound will become so 
faint that it cannot be heard, even when the ear is 
placed close to the receiver. 


ACOUSTICS. 


155 


There is perfect silence in a vacuum. No sound 
can therefore be transmitted to the earth from the 
regions of space. The 
movements of the heavenly 
bodies are noiseless. In 
the expressive language of 
David, “ Their voice is not 
heard.” In elevated re¬ 
gions sounds are dimin¬ 
ished in loudness. The 
explosion of a pistol on 
Mont Blanc is said to re¬ 
semble that of an ordinary 
fire-cracker; and it is diffi¬ 
cult to continue a conver¬ 
sation, as the voice must 
be raised so far above its 
natural pitch. The reverse 
of this takes place when 
persons descend into deep 
mines or in diving-bells. 

The sounds then become startlingly distinct, and the 
workmen are compelled to talk in whispers. 

The Velocity of Sound depends on the elasticity 
and density of the medium through which it passes. 
The higher the elasticity, the more promptly and rap¬ 
idly the motion will be transmitted, since the elastic 
force acts like a bent spring between the molecules : 
the greater the density, the more molecules to be 
set in motion, and hence the slower the transmission. 


Fig. 114. 









56 


NATURAL PHILOSOPHY. 


Sound travels through the air (at the freezing-point) 
at the rate of 1,090 feet per second. —A rise in temper¬ 
ature diminishes the density of the air, and thus 
sound travels faster in warm and slower in cold air. 
A difference of 1° F. makes a variation of about 1 
foot in velocity. 

Sound travels through water at the rate of 4,700 feet 
per second —Water is denser than the air, and for that 
reason sound should travel in it much more slowly; 
but its elasticity, which is measured by the force re¬ 
quired to compress it, is so much greater, that the 
rate is quadrupled. 

Sound travels through solids faster than through air. 
—This may be nicely illustrated by placing the ear 
close to the horizontal bar at one end of an iron 
fence, and having a person at the other end strike 
the fence a smart blow. Two successive sounds will 
reach the ear—one through the metal, and afterward 
another through the air. In some experiments made 
by Biot, when a bell was struck at the end of an iron 
tube 3,120 feet long, 2J seconds elapsed between the 
two sounds. This would make the velocity in iron 
nearly ten times that in air. 

Sounds travel ivith the same velocity. —Under ordi¬ 
nary circumstances we see that this must be true. A 
band is playing at a distance, yet the harmony of the 
different instruments is preserved. The soft and loud, 
high and low notes reach the ear at the same time. 
It has been said that the “ heaviest thunder trav¬ 
els no faster than the softest whisper.” This is not 


ACOUSTICS. 


157 


verified by careful investigations. Mr. Mallet found 
that in blasting with a charge of 2,000 lbs., the ve¬ 
locity was 967 feet per second, while with a charge of 
12,000 lbs. it was increased to 1,210 feet per second. 
Capt. Parry in his Arctic travels made a similar 
observation. He states that on a certain occasion 
when at a considerable distance, the sound of the 
sunset-gun reached his ears before the officer’s word 
of command to fire, proving that the report of the 
cannon travelled sensibly faster than the sound of 
the voice. 

The velocity of sound may he used to determine dis¬ 
tances. —Light travels instantaneously as far as all 
distances on the earth are concerned. Sound moves 
more slowly. For this reason we see a chopper 
strike with his axe, and a moment elapses before we 
hear the blow. If the time that intervenes is one 
second, we know that the distance is about 1,090 feet. 
By means of the second-hand of a watch or the 
beating of our pulse, we can count the seconds that 
elapse between a flash of lightning and the peal 
of thunder which follows. Multiplying the velocity 
of sound by the number of seconds, we have the dis¬ 
tance of the thunderbolt. 

The Intensity of Sound depends upon the ampli¬ 
tude of the vibrations. —The amplitude is the dis¬ 
tance the molecules swing to and fro. As in a 
pendulum, the greater the amplitude, the greater 
the velocity. In momentum, we found that the force 
of a striking body depends upon its weight and its 


>58 


NATURAL PHILOSOPHY. 


velocity. Just so, if one sound appears to us louder 
than another, it is because the air molecules hit the 
ear-drum with greater force. On the top of a moun¬ 
tain, because of the rareness of the atmosphere, there 
are fewer molecules to strike the ear; hence, ac¬ 
cording to the principles of momentum, the blow 
will be less intense. 

The intensity of sound diminishes as the square of the 
distance increases .*•—This is the natural effect of the 
expanding of the sound-wave, which proceeds in the 
form of a sphere. The larger the sphere, the greater 
the number of air particles to be set in motion, and 
hence the feebler their vibration. The surfaces of 
spheres are proportional to the squares of their radii. 
The radii of sound-spheres are their distances from 
the centre of disturbance. Hence the force with 
w r hich the molecules will strike our ear decreases as 
the square of our distance from the sounding body. 

Speaking-tubes conduct sound to distant rooms be¬ 
cause they prevent the waves from expanding and 
losing their intensity. Biot found that a conversa¬ 
tion in a low tone could be kept up through a Paris 
water-pipe 3,120 feet long. He says that “it was so 

* The same proportion obtains in Gravitation, Sound, Light, 
and Heat. We have seen how the pendulum is based upon tha 
force of gravity, and reveals the laws of falling bodies. Now we 
find that the pendulum, and even the principles of reflected mo¬ 
tion and momentum, are linked with the phenomena of sound. 
As we progress further, we shall find how Nature is thus inter¬ 
woven everywhere with proofs of a common plan and a common 
Author. 



ACOUSTICS. 


*59 


easy to be heard, that the only way not to be heard 
was not to speak at all.” A communication could be 
made in this manner even between two villages. The 
ear-trumpet acts by collecting waves of sound and re¬ 
flecting them into the ear. The speaking-trumpet is 
often explained on the same principle as the speak¬ 
ing-tube. A more rational theory is, that the sound 
of the voice is strengthened by the vibrations of the 
air in the tube. 

Refraction of Sound.— When a sound-wave goes 
obliquely from one medium to another, it is bent out 
of its direct course. It may even, like light, be 
passed through a lens and brought to a focus^ B is 
a thin rubber balloon, filled with carbonic acid gas ; 
to is a watch, and /' is a glass funnel which as- 


Fig. 115. 



sists in collecting the wave at /, where the ear is 
placed. By moving the head, a point will soon 
be found where the ticks of the watch can be heard 
very distinctly, while outside of it they are inau¬ 
dible. 










! 6o NA TVRAL PHILOSOPHY. 

Reflection of Sound.— When a sound-wave strikes 
against the surface of another medium, a portion 
goes on while the rest is reflected. 

The law which governs Reflected Sound is that of 
Reflected Motion ;—the angle of incidence is equal to 
that of reflection. Tyndall relates that a bell on a 
distant eminence in Heligoland failed to be heard 
in the town. A reflector was therefore placed behind 
it, so as to throw the sound-waves in the direction of 
the long sloping street. This caused every stroke 
to be distinctly audible. Domes and curved walls 
and ceilings act in the same manner as mirrors in 
the reflection of sound. Sir John Herschel relates 
an amusing illustration of this fact. A confes¬ 
sional in a cathedral in Sicily was so situated that 
the whispers of the penitents were reflected by the 
curved roof and brought to a focus at a distant part 
of the edifice. This point was accidentally discov¬ 
ered by a gentleman, who amused himself and his 
friends by listening to utterances intended for the 
ear of the priest alone. One day, however, his wife 
was the penitent, and both he and his friends were 
thus made acquainted with family secrets which 
were as new to himself as they were the reverse of 
amusing. “ The Ear of Dionysius” was a dungeon 
in Syracuse, so constructed as to convey to the ears 
of that tyrant every word spoken by its unfortunate 
inmates. Whispering galleries are commonly made 
of an elliptical form. Two persons, standing at the 
foci with their backs to each other, can thus carry on 


ACOUSTICS. 


161 


a conversation in whispers which are entirely un¬ 
noticed by those between them. Sound-waves have 
been brought to a focus by the mainsail of a vessel 
having accidentally taken a concave form ; in this 
manner a bell was once heard 100 miles out at sea. 

Decrease of Sound by Reflection .—If we strike the 
bell represented in Fig. 114, we find a great differ¬ 
ence between its sound under the glass receiver and 
in the open air. Floors are deadened with tan-bark 
or other fine material; since, as the sound-wave passes 
from each particle to the next of the unhomogene- 
ous mass, it becomes weakened by partial reflection. 
During a thunder-storm the air is of such varying 
density that thunder-peals are never heard at a dis¬ 
tance corresponding to their violence. For the same 
reason, the roar of cannon on a field of battle is not 
noticeable, and the day has often been lost within 
% short distance of the reserves of the defeated 
army, which were waiting for the sound of artillery 
to call them to the scene of action. The air at 
night is more homogeneous, and hence sounds are 
heard more clearly and farther than in the daytime. 
In foggy weather sounds suffer innumerable reflec¬ 
tions from the mist, and are soon destroyed. 
{Tyndall.) 



Resonance .—If the reflecting surface be very near, 
the reflected sound will join the direct one .and 
strengthen it. This effect is termed a resonance. It 
accounts for the well-known fact that a speaker can 
bo heard much more easily in a close room than in 


162 


NATURAL PHILOSOPHY. 


the open air. A smooth wall back of the stand 
re-enforces the voice in the same manner. The old- 
fashioned sounding-boards were by no means ineffi¬ 
cient, however inelegant may have been their appear¬ 
ance. Shells, by their peculiar convolutions, reflect 
and augment the various sounds which fill even the 
stillest air. As we hold them to our ear, they are 
poetically said to “ repeat the murmurs of their 
ocean-home.” Furniture and wall-hangings break 
up the resonance of a room ; and thus our footsteps 
in unfurnished dwellings sound startlingly distinct. 
Echoes are produced where the reflecting surface is 
so distant that we can distinguish the reflected 
sound from the direct one. If the sound be short 
and quick, this requires at least 56 feet; but if it be 
an articulate one, 112 feet are necessary. No one 
can pronounce or hear distinctly more than five syl¬ 
lables in a second; 1,120 ft. -r- 5= 224 ft.* If the wave 
travel 224 feet in going and returning, the two 
sounds will not blend, and the ear car^ detect a dis¬ 
tinct interval between them. A person speaking in 
a loud voice in front of a mirror, 112 feet distant, 
can distinguish the echo of the last syllable he utters; 
if twice that, or 224 feet, the last two syllables, etc. 
Places where good echoes may be heard abound in 
every locality. When several parallel surfaces are 
properly situated, the echo may be repeated back- 


* This-calculation supposes the sound to travel at the rate of 
1,120 feet per second. 



ACOUSTICS. 


163 

ward and forward in a surprising manner. At 
Woodstock, England, is one which repeats 17 sylla¬ 
bles by day and 20 by night. The reflecting surface 
is distant about 2,300 feet; a quick,sharp ha! will 
come back a ringing ha, ha, ha ! The echo is often 
softened, as in the Alpine regions, where it warbles a 
beautiful accompaniment to the shepherd’s horn. 

The difference between noise and music is only 
that between irregular and regular vibrations. What¬ 
ever may be the cause which sets the air in motion, 
if the vibrations be uniform and rapid enough, the 
sound is musical. If the ticks of a watch could be 
made with sufficient rapidity, they would lose their 
individuality and blend into a musical tone. “ The 
puffs of a locomotive are slow on first starting, but 
they soon increase so as to be almost incapable of 
being counted. If the puffs could reach 50 or 60 a 
second, the approach of an engine would be her¬ 
alded by an organ-peal of tremendous power.” 

Nothing can be imagined to be more purely a 
noise than the rattling of a cab over a stony street. 
The pavement of London is composed of granite 
blocks, four inches in width. A cab-wheel jolting 
over this at the rate of eight miles per hour produces 
a succession of 35 distinct sounds per second. These 
link themselves together into a soft, deep musical 
tone, that will bear comparison with notes derived 
from more sentimental sourc'es. {Houghton.) 

Pitch.— If we hold a card against the cogs of 
the wheel in the apparatus shown in Fig. 32, when 


NATURAL PHILOSOPHY. 


164 



turned rapidly we shall obtain a pure, clear tone; and 
the faster the wheel is revolved, the shriller the tone, 
or the higher the pitch. Hence we conclude that 
Pitch depends on the ra¬ 
pidity of the vibrations. . 

HOW TO FIND THE NUM* 
BEE OF WAVES IN A MUSI¬ 
CAL sound.— This is de¬ 
termined by means of an 
instrument called the Si¬ 
ren. C is a cylindrical 
box ; t, a pipe for admit¬ 
ting the air ; a b, a plate 
pierced with four series 
of holes, containing 8, 
10, 12, and 16 orifices 
respectively; m, n, o, p 
are stops for closing any 
series at pleasure. The 
vertical rod p is bevelled 
at p' so as to turn in the 
socket x; d e is a disk 
pierced with holes cor¬ 
responding to those in 
the lower plate, over 
which it is made to re¬ 
volve. At s is an end¬ 
less screw, which, as the 
axis p revolves, causes two wheels to rotate, and 
thus turns the hands upon the dial (Fig. 118). On 





























ACOUSTICS. 


65 


this we can see at any moment the number of 
revolutions made by the upper disk. The holes 
in a b and d e are inclined to each other, so that, 
when a current of 
air is forced in at t , 
it passes up through 
the openings in the 
lower disk, and strik¬ 
ing against the sides 
of those in the upper 
disk, causes it to re¬ 
volve. As the upper 
disk turns round, it 
altern ately opens and 
closes the orifices in 
the lower disk, and 
thus converts the 
steady stream of air 
into uniform puffs. 

At first they succeed 
each other so slowly 
thai^ they may be 
easily counted. At 
last, as the motion 
increases, they link 
themselves together, 
and burst into a full, 
melodious note. As the velocity augments, the pitch 
rises, until the music becomes so shrill as to be painful. 
Diminish the speed, and the pitch falls immediately. 





















































166 


NA TUBAL PHIL OSOPHY. 



Let us now see how the Siren is used to determine 
the number of vibrations in any sound. Force the 

air through it stead¬ 
ily until the tone is 
brought to any re¬ 
quired pitch. Find 
on the dial, at the 
end of a minute, the 
number of revolu¬ 
tions made by the 
disk. When the row 
containing ten holes 
is open, and the tone 
C 2 , it will indicate 1,536. There were ten puffs 
of air, or ten waves of sound, in each revolution. 
1,536 X 10 = 15,360. Dividing this by 60, we have 
256 as the number per second. When the inner 
and outer rows of holes are opened, the ear im¬ 
mediately detects the difference of an octave be¬ 
tween the two sounds. The one containing 8 pro¬ 
duces the lower, and the one containing 16 the 
higher tone. Hence we conclude that an octave 
of any tone is caused by double the number of 
vibrations. 

How to find the length of the wave in a musical 
sound .—Suppose the air in the last experiment was 
of such a temperature that the foremost sound-wave 
would have reached the distance of 1,120 feet in a 
second. In that space there were 256 sound-waves. 
Dividing 1,120 by 256, we have 4 feet 4 inches as the 



























ACOUSTICS. 


67 


length of each. We see from this that we find the 
wave-length by dividing the velocity of sound by 
the number of vibrations per second. As the pitch 
is elevated by rapidity of vibration, we readily per¬ 
ceive that the low tones in music are produced by 
the long waves and the high tones by the short 
waves. An experiment illustrative of this can be 
made when an express-train passes a railroad sta¬ 
tion. As the engine approaches us, the waves from 
the whistle are shortened by the rapid motion, and 
as it recedes, are lengthened; the pitch of the 
whistle will therefore be raised as the train comes 
in, and be lowered as it goes out. The same result 
may be detected if a person in a high swing pro¬ 
duces, while in swift motion, a continuous musical 
tone upon some instrument. 

Application to any Musical Sound .—Whenever notes 
from any two sources are in unison, they are pro¬ 
duced by the same number of vibrations. If the 
string of a violin, the cord of a guitar, the parch¬ 
ment of a drum, and the pipe of an organ, produce 
the same musical tone, it is because the vibrations 
in all are isochronous. “ If a voice and a piano 
execute the same music, the steel strings of the 
piano and the vocal cords of the singer vibrate 
together and send out sound-waves of the same 
length.” In order, then, to determine the number 
and length of the sound-waves produced by a sono¬ 
rous body, we have only to bring its sound and that 
of the siren in unison. In this way it has been found 


NATURAL PHILOSOPHY. 


168 


that the wings of a gnat flap, in flying, at the rate 
of 15,000 times per second. The waves produced 
jby a man’s voice in ordinary conversation are from 
eight to twelve feet in length, and by a woman’s 
voice from two to four feet. ( Tyndall .) 

Super-position op sound-waves. —(See Wave Mo¬ 
tion, p. 128.)—The air may transmit sound-waves from 
a thousand instruments at once. If the condensa¬ 
tion of one wave meet the condensation of another, 
it will augment the sound, the condensations becom¬ 
ing more condensed and the rarefactions more rare¬ 
fied by their coincidence. If, on the other hand, the 
condensation of one meet the rarefaction of the 
other, the result will be changed ; one wave motion 
will be striving to push the air molecules forward, 
and the other to urge them backward. Thus, if 
they meet in exactly opposite phases and the two 
forces are equal, they will balance each other and 
silence will ensue. Thus a sound added to a sound 
may produce silence. In the same way, two motions 
may produce rest; two lights may cause darkness ; and 
two heats may produce cold. 


Fig. 119. 


Il illllllllllflilllllllllllliyilifllif lli nttDhliU l ll l i n n illTrTiflTfTrnffTTTTnTllilMI Hyi l il l llllllliliii l illllll ll lll lHIIIlMBBMJIIIlllll il lll iiillilili im III iI IIIiIWBBiIIII IIII II IiIIIiIII iIIIIIIIIIIIIIIIMBIIIIMIIIIIIIII 

! : j ■ |;:||| : I; ! ^ ■ J UIl':. : : , , JliBsil: 1 hillllB 


Suppose we have two tuning-forks, A and B, 
placed a wave-length apart, and vibrating in unison. 



ACOUSTICS. 


169 

The waves from the two will coincide, as represented 
by the light and dark shades in the figure. The 
same would occur if they were placed at any num¬ 
ber of wave-lengths apart. If they are a half wave¬ 
length apart, the condensation of A coincides with 
the rarefaction of B, and vice versa. The effect is 
represented by the uniformity of the shading in Fig. 
120. This is termed interference of sound-waves . 


Fig. 12 a 

B A. 



There are positions in which the prongs of a 
tuning-fork interfere with each other so as to pro¬ 
duce silence. If we strike the fork and turn it 
slowly around before the ear, we shall find four points 
where the interference of the sound-waves entirely 
neutralizes the vibrations. 

Vibrations of Cords.—L et a b be a stretched 


Fig. 121. 
d 



cord made to vibrate. The motion from e to d and 
back again is termed a complete vibration; that 
from e to d alone, is a half-vibration. The intensity 
of the sound depends on the width of e d^ i. e., the 
8 























NATURAL PHILOSOPHY 


170 

amplitude of the vibration. It is, however, very 
weak, on account of the small amount of air a sim- 
j)le cord can set in motion. The laws which govern 
the number of vibrations, and hence the pitchy are 
investigated by means of an instrument known as 
the Sonometeb. It consists of two cords stretched 
by weights at P, across two fixed bridges, A and B. 


Fig. 122. 



D is a movable bridge, which serves to lengthen or 
shorten the cords at pleasure. Beneath is a wooden 
box which receives the vibrations of the cords and 
communicates them to the air within. This is the 
real sounding body. 

1st Law. The number of vibrations per second in¬ 
creases as the length of the cord decreases. —Let the 

cord be caused to vibrate, and we shall hear the note 
of the entire string. Now place the movable bridge 
D at the centre of the cord, and we shall obtain a 
sound the octave of the former. Thus by taking 













ACOUSTICS. 


71 


one-lialf the length of the cord we double the num¬ 
ber of vibrations. If an entire cord makes 20 
vibrations per second, one-half will make 40, and 
one-third, 60. The violin or guitar player elevates 
the pitch of any string of his instrument by moving 
his finger, and thus shortening the length of the 
vibrating portion. In the piano, harp, etc., the 
long and short strings produce the low and high 
notes respectively. 

2d Law. The number of vib'ations per second 
increases as the square-root of the tension .—The cord 
when stretched by 1 lb. gives a certain tone : to 
double the number of vibrations and obtain the 
octave requires a weight of 4 lbs. All stringed in¬ 
struments are provided with keys, by means of which 
the tension of the cord and the corresponding pitch 
may be increased or diminished. 

Bd Law. The number of vibrations per second de¬ 
creases as the square-root of the weight of the cord in¬ 
creases .—If two strings of the same material be 
equally stretched, and one have four times the weight 
of the other, it will only vibrate half as often. In 
the violin the bass notes are produced by the thick 
strings. In the piano the result is obtained by coil¬ 
ing fine wire around the heavy strings. 

Nodes. —In the experiments just named, the cord 
was shortened by means of a movable bridge which 
held it firmly at the centre. If, instead, we simply 
rest the feather-end of a goose-quill lightly on the 
string, and then draw the bow over one-half, it will 


172 


NATURAL PHILOSOPHY 


vibrate in two portions and will give the octave as 
before. Remove the feather, and it will continue to 


Fig. 123. 



vibrate in two parts and to yield the same tone. 
We can show that the second half vibrates by sim¬ 
ply placing across the middle of that portion of the 
wire a little paper rider. On drawing the bow the 


Fig. 124. 



rider will be thrown off. Hold the feather so as to 
separate one-third of the string and cause it to 
vibrate. The remainder of the cord will vibrate 
in two segments. When the feather is removed, the 









ACOUSTIC'S. 


73 


entire cord will vibrate in three different parts of 
equal length, separated by stationary points called 
nodes. This may be shown by placing on the wire 
three riders; the one at the node will remain, while 
the others will be thrown off. In the same manner 
the cord may be divided into any number of equal 
vibrating segments and stationary nodes. 

Acoustic Figures .—Sprinkle some fine sand on a 
glass or metal plate. Place the finger-nail on one 


Fig. 125. 



edge to stop the vibration at that point, as the 
feather did in the last experiment, and draw the bow 
lightly across the opposite edge. The sand will be 
tossed away from the various parts of the plate and 
will collect along two nodal lines, which divide the 
large square. It is wonderful to see how the sand 
will seemingly start into life and dance into line at 
the touch of the bow. Fig. 126 shows some of the 
beautiful patterns obtained by Chladni. 












ACOUSTICS. 


175 

Harmonics or overtones. —Even when a cord is caused 
to vibrate in its full length, it separates into parts at 
the same time. Thus we have the full or fundamen¬ 
tal note of the entire string; and superposed upon 
that, the higher notes produced by the vibrating 
parts. These are called overtones or harmonics. 
The overtones vary in different instruments. The 
mingling of the two classes of vibrations determines 
the quality of the sound, and enables us to distin-i 
guish the music of different instruments. 

Nodes of a Bell. —Let the heavy circle in Fig. 1#7, 
represent the circumference of a bell when at rest. 
Let the hammer strike at 
a, b, c , or d. At one moment 
as the bell vibrates it will 
form an oval with a b , at 
the next with c d for its 
longest diameter. When 
it strikes its deepest note, 
the bell vibrates in four 
segments, with n y n t n, n f as 
the nodal points, whence 
nodal lines run up from the edge to the crown of tha 
bell. It tends, however, to divide into a greater 
number of segments, especially if it is very thin, 
and so to produce a series of harmonic sounds. 
These overtones, which follow the deep tones ofr r the 
bell, are frequently very striking, even in a common 
call-bell. 

Nodes of a Sounding-board. —The case of the violin 


Fig. 127. 




176 


NATURAL PHILOSOPHY. 


or guitar is composed of thin wooden plates which 
divide into vibrating segments, separated by nodal 
lines according to the pitch of the note which is 
being played. The enclosed air vibrating in unison 
with these, re-enforces the sound and gives it fulness 
and richness. The sounding-board of the piano 
acts in a similar manner. 

The Musical Scale. —The tone produced by the 
vibrations of an entire string is called its fundamen¬ 
tal sound. The various sounds of the scale above 
this are given by the parts of the string indicated by 
the fractions 

C, D, E, P, G, A, B, C. 

l % 4 A 8 A a / 8 s / 6 Vis V. 

As the number of vibrations varies inversely as 
the length of the cord, we need but to invert these 
fractions to obtain the relative number of vibrations 
per second ; thus, £, f, f, f, -f, 1 /, 2. Reduced to a 
common denominator, their numerators are propor¬ 
tional, and we have the whole-numbers which repre¬ 
sent the relative rates of vibration of the notes of 
the scale, viz.: 

24, 27, 30, 32, 36, 40, 45, 48. 

The number of vibrations corresponding to the 
different letters is, 

C, D, E, F, G, A, B, C. 

128, 144, 160, 170. 192, 214, 240, 256. 

Wind Instruments produce musical sounds by 
means of enclosed columns of air. Sound-waves 
run backward and forward through the tube and act 
on the surrounding air like the vibrations of a cord. 
The sound-waves in organ-pipes are set in motion 


ACOUSTICS. 


177 


either by means of fixed mouth-pieces or vibrating 
reeds. The air is forced from the bellows into the 

Fig. 128. 



tube P, through the vent i, and striking against the 
thin edge a, produces a flutter. The column of air 
above, being thus thrown into vibration, re-enforces 
the sound and gives a full musical tone. The 
length of the pipe determines the pitch. The varia¬ 
tion in the quality of different wind-instruments is 
caused by the mingling of the harmonics with the 
fundamental tone. In the flute, for example, the 
8 * 





























178 


NATURAL PHILOSOPHY. 


vibrating column of air may be made to break up 
into vibrating segments with stationary nodes, by 
merely varying the force of the breath. 

Sympathetic Yibeations. —Stand near a piano 
and produce a musical tone with the voice, and you 
will find that a certain wire selects that pulse of sound 
and responds to it. Change the pitch, and the first 
string ceases, while another replies. If a hundred 
tuning-forks of different tones be made to sound at 
the foot of an organ-pipe, it will choose the one to 
which it is able to reply, and respond to that alone. 
Two* clocks set on one shelf or against the same 
wall, affect each other. Watches in the shop-win¬ 
dow keep better time than when carried singly. 

The Ear. —Fig. 129 is a sketch of the ear, drawn 


from a model. The 
small bones are 
much magnified, in 
order to give a 
distinct idea of 
their shape. A F 
represents a rear 
view of the outer 
ear. The sound¬ 
wave passes into 
the auditory canal, 
which is about one 
inch in length, and 
striking against the 


Fig. 129. 



tympanum or drum, E, which closes the orifice 


ACOUSTICS. 


179 


of the external ear, throws this membrane into 
vibration. Next, the series of small bones, a, b, c, 
called respectively, from their peculiar form, the 
hammer , anvil, and stirrup , conduct it to the inner 
ear, which is termed, from its complicated structure, 
the labyrinth. This is filled with liquid, and contains 
the semi-circular canals, B, and the cochlea {snail- 
shell), C, which receive the vibrations and transmit 
them to the auditory nerve, the fine filaments of which 
are spread out to catch every pulsation of the sound¬ 
wave. The middle ear, which contains the chain of 
small bones, is a simple cavity about J inch in diam¬ 
eter, filled with air. It communicates with the 
mouth by means of the Eustachian tube, D. Within 
the labyrinth are also fine, elastic hair-bristles and 
crystalline particles among the nerve-fibres, wonder¬ 
fully fitted, the one to receive and the other to pro¬ 
long the vibrations; and lastly, a lute of 3,000 micro¬ 
scopic strings, so stretched as to vibrate in unison 
with any sound. The Eustachian tube is generally 
closed, thus cutting off the air in the inner cavity 
from the external air. If at any time the pressure 
of the atmosphere without becomes greater or less 
than that within, the tympanum feels the strain, 
pain is experienced, and partial deafness ensues. 
A forcible concussion frequently produces in this 
way a temporary deafness. In the act of swallow¬ 
ing, the tube is opened and the equilibrium restored. 
We may force air into the cavity of the ear by 
closing our mouth and nose, and forcibly expiring 


! g Q NA TURAL PHILOSOPHY. 

the air from our lungs. This will render us insensi¬ 
ble to low sounds, as the rumble of a railway-train, 
while we can hear the higher ones as usual. 

Limits of Hearing.— Helmholtz fixes the lowest 
limit of musical sounds at 16 vibrations per second,* 
and the highest at 38,000. Below this number the 
pulses cease to link themselves together, and be¬ 
come distinct sounds. The range of the ear is thus 
about eleven octaves. The practical range of music 
is, however, only about seven octaves. The capacity 
to hear the higher tones varies in different persons. 
A sound which is entirely audible to one may be 
utter silence to another. Some ears cannot distin¬ 
guish the squeak of a bat or the chirp of a cricket, 
while others are acutely sensitive to these shrill 
sounds. Indeed, the auditory nerve seems generally 
more alive to the short quick vibrations than to the 
long slow ones. The whirr of a locust is much more 
noticeable than the sighing of the wind through the 
trees. To this, however, there are noticeable excep¬ 
tions. The author knows of a person who is en¬ 
tirely insensible to the higher tones of the voice, but 
acutely sensitive to all the lower ones. Thus on 
one occasion, being in a distant room, she did not 
notice the ringing of the bell announcing dinner, 
but heard the noise the bell made when returned 


* A tone produced by about 32 vibrations per second may be 
made by inserting the finger lightly into the ear, bringing at the 
same time the muscles of the hand into strong contraction. A 
sound will be heard which is as deep as the toll of a cathedral bell. 



ACOUSTICS. 


181 


to its place on tlie shelf. A continuous blast of air 
has no effect to produce sound. The rush of the 
grand aerial rivers above us we never hear. They 
flow on ceaselessly but silently in the upper regions 
of the air. A whirlwind is noiseless. Let, however, 
the great billows strike a tree and wrench it from 
the ground, and we can hear the secondary shorter 
waves which set out from the struggling limbs and 
the tossing leaves. 

Our unconsciousness is no proof of the absence 
of sound. There are, doubtless, sounds in Nature 
of which we have no conception. Could our sense 
be quickened, what celestial harmony might thrill 
us ! Professor Cooke beautifully says : “ The very 
air around us may be resounding with the halle¬ 
lujahs of the heavenly host, while our dull ears 
hear nothing but the feeble accents of our broken 
prayers.” 

The ability of the ear to detect and analyze sound 
is wonderful beyond all comprehension. Sound¬ 
waves chase each other up and down through the 
air, superposed in entangled pulsations, yet a cylin¬ 
der of the air not larger than a quill conveys them 
to the ear, and each string of that wonderful harp 
selects its appropriate sound, and repeats the music 
to the soul within. Though a thousand instruments 
be played at once, there is no confusion, but each is 
heard, and all blend in harmony. 

The tendency of Nature to Music. —“ Friction,” 
says Tyndall, “ is rhythmic.” A bullet flying through 


✓ 




182 


NATURAL PHILOSOPHY. 


the air sings softly as a bird. The limbs and leaves 
of trees murmur as they sway in the breeze. The 
rumble of a great city, all the confused noises of na¬ 
ture when softened by distance, are said to be on one 
pitch—the key of F. Falling water, singing birds, 
sighing winds, everywhere attest that the same Di¬ 
vine love of the beautiful which causes the rivers 
to wind through the landscape, the trees to bend in 
a graceful curve—the line of beauty—and the rarest 
flowers to bud and blossom where no eye save His 
may see them, delights also in the anthem of praise 
which Nature sings for His ear alone. 

Sensitive Flames.— Flames are frequently ex¬ 
tremely sensitive to certain sounds. At an instru¬ 
mental concert the gaslights often vibrate in unison 
with certain pulsations of the sound which they seem 
to select. This is most noticeable when the pres¬ 
sure of gas is so great that the flame is just on the 
verge of flaring, and the vibration of the sound¬ 
wave is sufficient, as it were, to “ push it over the 
precipice.” If we turn on the gas, in a common 
fish-tail burner, we reach a point where a shrill 
whistle will produce the same effect as increased 
pressure of the gas, and cause the jet to thrust out 
long, quivering flames. Prof. Barret, of London, de¬ 
scribes a peculiar jet which was so “ sensitive that 
it would tremble and cower at a hiss, like a human 
being, and even beat time to the tic kin g of a 
watch.” 

Singing Flames. —If we lower a glass tube over a 


ACOUSTICS. 


183 



small jet of gas, we soon reach a point where the 
flame leaps spontaneously into song. At first the 
sound seems far re¬ 
mote, but gradually 
approaches until it 
bursts into a shrill 
scream that is al¬ 
most intolerable. 

The length of the 
tube and the size 
of the jet determine 
the pitch of the 
note. If we raise 
the tube to a point 
where it is just ready 
to sing, we shall find 
that it will respond 
to the voice when 
the proper note is 
struck.* 

The flame, owing 
to the friction at the 
mouth of the pipe, 
is thrown into vibration. The air in the tube, being 
heated, rises, and not only vibrates in unison with 
the jet, but, like the organ-pipe, selects the tone 
which is adapted to its length, and in part governs 
the pulsations of the flame. 


* See Rev. Chem., p. 55. The jets are made by drawing out 
glass tubing to a fine point over a spirit-lamp. The length of the 
tube may be varied, as in the figure, by means of a paper tube. 

















^ Y0 / /k\sJjL 


184 


NATURAL PHILOSOPHY. 


Practical Questiotis 1. Why cannot the rear of a long column of sol¬ 
diers keep time to the music in front ? 2 . Three minutes elapse between the 
flash and report of a thunderbolt; how far distant is it? 3 . Five seconds 
expire between the flash and report of a gun ; what is its distance ? 4 . Sup-_/ 

pose a speaking-tube should connect two villages ten miles apart; how long < ' 1 
would it take the sound to travel that distance ? 5. The report of a pistol-^ I CL (] 

shot was returned to the ear from the face of a cliff in four seconds ; what ^ 
was the distance? 6. What is the cause of the difference between the voice 
of man and woman? A base and a tenor voice ? 7. What is the number of /Q -Q 

vibrations per second necessary to produce the fifth tone of the scale of C? ' '* 

8. What is the length of each sound-wave in that tone when the temperature £ fLJ 
is at zero? 9 . What is the number of vibrations in the fourth tone above. 
middle C? 10. A meteor of Nov. 13,1868, is said to have exploded at a height 
of 60 miles ; what time would it have required for its sound to reach the earth ? ^ i un 

11. A stone was let fall into a well, and in four seconds was heard to strike 
the bottom; how deep was the well? 12 . What time will it require for a 
sound to travel five miles in the still water of a lake ? 13. How much loudeiv- : / 

will be the report of a gun to an observer at a distance of 20 rods than to onefc> 1 
at half a mile ? 14. Does sound travel faster at the foot or at the top of a 
mountain ? 15. Why is an echo weaker than the original sound feV.l 6. Why 
is it so fatiguing to talk through a speaking-trumpet ? 


I 


I 


Curious Facts in Sound. —Silliman says the roar of cannon has been heard 
at a distance of 250 miles by putting the ear to the ground. In Capt. Parry’s 
third Expedition, Lieut. Forster carried on a conversation with a man at a 
distance of 1 i miles. The sentinel’s “ All’s well” has been heard from Old to 
New Gibraltar, a distance of 10 miles. The cannonading at the battle of Jena 
was heard at Dresden, 92 miles away. The celebrated echo of the Metelli a* 
Rome was capable of repeating the first line of the iEneid 8 times distinctly 
In Fairfax County, Va., is an echo which will return 20 notes played on a 
flute, but supplies the place of some notes with their thirds, fifths, or octaves. 
Sir John Herschel says the tick of a watch may be heard from one end of the 
Abbey Church of St. Albans to the other. At Carisbrook Castle, in the Isle 
of Wight, is a well 210 feet deep and 12 feet wide. It is lined with smooth 
masonry. When a pin is dropped into the well it is_distinctly heard to strike 
the water. In certain parts of the Colosseum at London the tearing of paper 
sounds like the patter of hail, while a single exclamation comes back a peal 
of laughter. 

A tired bee hums on E, while in pursuit of honey it hums contentedly on 
A. The common horse-fly, when held captive, moves its wings 335 times a 
second; a honey-bee, 190 times. 

Youmans says it is marvellous how slight an impulse throws a vast amount 
of air into motion. We can easily hear the song of a bird 500 feet above us. 
For its melody to reach us it must have filled with wave-pulsations a sphere 
of air 1,000 feet in diameter, or set in motion 18 tons of the atmosphere. 




S7 y * - 

■ - 0 r >0 4 








The sanbeam comes to the earth as simply motion of ether-waves, yet it is 
the only source of beauty, life, and power. In the growing plant, the burning 
«oal, the flying bird, the glaring lightning, the blooming flower, the rushing 
engine, the roaring cataract, the pattering rain—we see only vailed manifesta¬ 
tions of this one all-energizing fore*. 

























. 






























































• 4 








' 2 . »-■ ‘ij L g 
















I 
















OPTICS. 

Optics treats of Light. 

Definitions. —A luminous body is one that emits 
light. A non-luminous body is one that reflects 
light, and is visible only in the presence of a lumi¬ 
nous body. A medium is any substance through 
which light passes. A transparent * body is one that 
offers so little obstruction to the passage of light 
that we can see objects through it. A translucent 
body is one that lets some light pass, but not 
enough to render objects visible through it. An 
opaque body is one that does not transmit light. A 
ray of light is a single line of light; it may be traced 
in a dark room into which a sunbeam is admitted. 


* Though we speak of transparent and opaque substances, 
these terms are merely relative. No body is perfectly transpar¬ 
ent, nor is any entirely opaque. Glass obstructs some light. 
It is said that if the atmosphere were 700 miles deep, no light 
would reach our eye. Deep-sea dredging has recently shown 
that light penetrates water to great depths, so that even the 
Atlantic cable may not lie in an abyss of utter darkness. On the 
other hand, gold, when beaten into leaf, becomes translucent, 
and appears of a faint green color; and horn, when scraped 
becomes semi-transparent 



188 


NATURAL PHILOSOPHY. 


by the floating particles of dust which reflect the 
light to the eye. 

The Visual Angle is the angle formed at the eye 
by rays coming from the extremities of an object. 
The angle A 0 B is the angle of vision subtended 


Pig. 131. 



by the object A B. The size of this angle varies 
with the distance of the body. A B and A' B' are 
of the same length, and yet the angle AO B' is 
much smaller than A 0 B, and hence A' B' will 
seem much shorter than A B. We estimate the dis¬ 
tance and size of objects in various ways, but the 
two are intimately connected, since we have by 
long experience learned to associate them. Know¬ 
ing the distance of an object, we determine its size 
immediately from the visual angle. We can vary 
the apparent size of any body at which we are look¬ 
ing, if by any means we increase or diminish this 
angle. This principle will be found of great impor¬ 
tance in the formation of images by mirrors and 
lenses. 

Laws of Light. —1. Light passes off from a lumi¬ 
nous body equally in every direction. 

2. Light travels through a uniform medium in 
straight lines. 



OPTICS. 


189 


3. The intensity of light decreases as the square, 
of the distance increases. 

The Velocity of Light is about 183,000 miles 
per second. This is so great that for all distances 
on the earth it is instantaneous. A sunbeam would 
girt the globe quicker than we can wink. This rate 
has been determined in various ways, but a most 
simple method is thus explained. The planet Ju¬ 
piter has four moons; as these pass around the 
planet, they are eclipsed from our sight at regular 
intervals. In the cut, let J represent Jupiter, e one 

Fig. 138. 



of the moons, S the sun, and T and t different posi¬ 
tions of the earth as it moves in its orbit around the 
sun. Homer noticed that when the earth was at T, 
the eclipse occurred 16 min. and 36 sec. earlier than 
when at t. He could account for this only on the 
supposition that it requires that time for the light 
to travel across the earth’s orbit. This distance is 
183,000,000 miles. Hence the velocity is about 
183,000 miles per second. 

The Undulatory Theory of Light. —There is 
supposed to be a fluid, termed ether , constituting a 


190 


NA TUBAL PHIL OSOPHY. 


kind of universal atmosphere, diffused throughout all 
space. It is so subtle that it fills the pores of all 
bodies, eludes all chemical tests, passes in through 
the glass receiver and remains even in the vacuum 
of an air-pump. A luminous body sets in motion 
waves of ether, which pass off in every direction. 
These move at the rate of 183,000 miles per second, 
and breaking upon the eye, give to us the impres¬ 
sion of sight. This etherial wave-motion is pre¬ 
cisely like that of sound, except that the vibrations 
are transverse (crosswise) to the line of direction. 
Thus, if we suppose a star directly overhead and a 
ray of light coming down to us, we should conceive 
that the particles which compose the waves are 
vibrating N. S. E. W., and toward every other point 
of the compass all at once. 

Reflection of Light. 

Diffused and Reflected Light.— -When light 
falls upon any surface, one portion is transmitted 
and another is reflected. The law is that of Motion 
and Sound— i. e., “ The angle of incidence is equal to 
the angle of reflection.” When the surface is 
rough, the multitude of little protuberances scatter 
the rays in every conceivable direction, and we can 
therefore see such a body from any point. This 
forms what we term diffused light. When the surface 
is smoothed and polished, the rays are more uni¬ 
formly reflected in particular directions, and, when 
we stand in the proper position, will bring to us the 


OPTICS. J g j 

images of other objects. We thus view all non- 
luminous objects by means of irregularly reflected 
(diffused) light, and images of objects by means 
of regularly reflected light. However, the most 
perfectly polished substance diffuses some light— 
enough to enable us to trace its surface; were it not 
so, we could not be aware of its existence. The de j 
ception of a mirror is oftentimes very nearly com¬ 
plete ; yet a little dust or vapor, increasing the irregu¬ 
lar reflection, will at once bring the surface to view. 

Beflection varies with the angle.— We notice 
this very clearly if we look at the images of objects 
in still water. Those which are near us are not as 
distinct as those on the opposite bank, because the 
rays from the latter strike the water more obliquely 
than the former, and so are more perfectly reflected 
to the eye. The image of the sun at mid-day is not 
so bright as when it is near the horizon. 

Mirrors. —All reflecting surfaces are termed mir¬ 
rors. These are of three kinds— plane , concave , and 
convex. The first has a flat surface ; the second, 
one like the inside, and the third, one like the outside 
of a watch-crystal. 

The general principle of mirrors is, that the image 
is always seen in the direction of the reflected ray as if 
enters the eye. 

Plane Mirrors. —Bays of light retain their rela¬ 
tive direction after reflection from a plane surface.* 


* The effect of the various mirrors is best understood if we 
draw a mirror of the kind under consideration, and then repre- 




192 


NATURAL PHILOSOPHY. 


An image seen in a plane mirror is therefore erect 
and of the same size as the object. It is, however, 
reversed right and left. 

Why the image is seen as far behind the mirror as 
the object is in front of it. —Let A B be an arrow held 
Fig. 133. in front of the mirror, 

M N. Kays of light 
from the point A 
striking upon the 
mirror at C, are re¬ 
flected, and enter the 
eye as if they came 
from a. Kays from B, 
in the same manner, seem to come from b. Since 
the image is seen in the direction of the reflected 
rays, it appears at a b, a point as far behind M N 
as the real arrow is in front of it. 

Why we can see several images of an object in a mir¬ 
ror. —Metallic mirrors form but a single image. If, 
however, we look obliquely at the image of a candle 


sent rays of the different classes, erect perpendiculars at the 
point of incidence and find the reflected rays. A little prac¬ 
tice of this kind will benefit more than any description. It will 
aid in drawing the perpendicular to a convex or concave sur¬ 
face, if we remember that it is always a radius of the sphere 
of which the mirror forms a part. A book held in various posi¬ 
tions before a common looking-glass should be used to illustrate 
the action of plane mirrors. Many of the effects of concave and 
convex mirrors may be seen on the inner and outer surface of a 
bright spoon, a call-bell, or a metal cup. Much instructive 
amusement may be obtained in the examination of the curious 
and grotesque figures thus revealed. 





OPTICS. 


l 9 3 


in a looking-glass, we shall often see several images, 
the first one very feeble, the next bright, and the 
others gradually diminishing in 
intensity. The ray from A is in 
part reflected to the eye from the 
glass at b, and gives rise to the 
image a; the remainder passes 
on and is reflected from the me¬ 
tallic surface at c, and coming to 
the eye forms a second image a'. 

The ray c d , when leaving the glass at d , loses a part, 
which is reflected back to form a third image. This 
ray in turn is divided to form a fourth, and so on. 


Fig. 134. 



Fig. 135. 



Images seen in water are symmetrical, but inverted. 
The reason of this is best understood by holding an 
9 











194 


NATURAL PHILOSOPHY. 


object in front of a horizontal looking-glass and no¬ 
ticing the angle at which the various rays must strike 
the surface in order to be reflected to the eye. "When 
the sun or moon is shining high in the heavens, we 
always see the image in the water at only one spot, 
while the rest of the surface appears dark. The light 
falls upon all parts, but the rays are reflected at the 
right angle to reach the eye from one point alone. 
Each observer sees the image at a different place. 
When the surface of the water is ruffled, a long tremu¬ 
lous line of light is reflected from the side of each tiny 
wave that is turned toward us. As each little billow 
rises, it flashes a gleam of light to our eye, and then 
sinking, comes up beyond, only to reflect another ray. 

A Concave Mirror tends to collect the rays of light. 
—The point where the reflected rays meet is termed 
the principalfocus (focus, a hearth). It is half way be¬ 
tween the mirror and the centre of curvature— i. e. f 
the centre of the hollow sphere of which the mirror 
is a part. In Fig. 136 we have parallel rays falling 
upon a mirror. C is the centre of curvature; F, 
the principal focus, half-way between A and C ; A F, 

the foeal 
distance; 
C B, C D, 
etc., radii of 
the sphere 
(perpendic¬ 
ulars, to find the angle of incidence); the angles 
H B C, G D C, etc., equal respectively to F B C, 




OPTICS . 


*95 


F D C, etc. A light held at C will have its rays 
brought to a focus at C; on the other hand, one at 
F will be reflected in a beam of parallel rays. 

Images formed by concave mirrors .—Hang a con¬ 
cave mirror against the wall, and stand closely to it 
between the mirror and the principal focus. The 
image is erect and much larger than life. Tbe ray a 
falls upon the mirror, is reflected and strikes the 

Fig. 187. 



eye as if it came from A. In the same manner b is 
seen at B. The visual angle is increased the nearer 
we approach the mirror, and hence the greater the 
magnifying power. We now walk back. When we 
reach the focus, the image becomes blurred, and 
finally disappears. We are in the position of the 
candle a b (Fig. 138), and the real image is behind 





l gQ NATURAL PHILOSOPHY. 

us at A B. A few of the parallel rays, however, 
enter the eye, and an indistinct image is formed. 
[Retiring still farther, we come to the centre of curva¬ 
ture. Here we find no distinct image, although 
portions of our figure, as we catch snatches of the 
rays which are forming the image A B, are magnified 
in the most uncouth and absurd manner. As we 

Fig. 138. 



continue to recede, we reach a point beyond the 
centre of curvature. Here we occupy the position 
A B (Fig. 138), and we see the image in the position 
a b, in front of the mirror inverted. It is inverted 
since the rays cross at the focus, and is smaller than 
life because the visual angle is diminished. The 
positions occupied by the two candles, a b and A B, 
are termed conjugate foci , since an object at either N 
point is brought to a focus at the other. 

Fig. 139. A Convex Mjh- 

ror tends to scatter 
the rays of light .— 
In the figure we 
notice how the 
parallel rays A D 
and B K are reflected in the diverging lines D E 





OPTICS. 


19 7 


and K H. An eye receiving these rays will perceive 
the image of A B at a 6, erect, and smaller than the 
object since the visual angle is diminished. 

Total Reflection.— When we look very obliquely 
into a pond, we cannot see the bottom, because 
the rays of light from below 
are reflected downward at the 
surface. If we look up into a 
glass of water, we shall see the 
upper surface gleaming like bur¬ 
nished silver. This effect occurs 
only when light passes at a 
definite angle from a denser to a 
rarer medium. It is termed total , since, unlike the 
other cases of reflection, all the light is bent back. 


Fig. 140. 



Refraction of Light. 

We have already seen that when a ray of light 
passes from one medium to another of different 
density, one portion is irregularly reflected, and by 
that means the surface is made visible; that, if the 
surface is smooth, the larger part is regularly re¬ 
flected, and in that way images of objects are seen. 
We now speak of the portion which passes on to the 
next medium, and which is refracted or bent out of 
its course. 

Illustrations of Refraction. —A spoon in clear 
tea appears bent. An oar dipping in still water 
seems to break at the point where it enters the 
water. Fish seem nearer the surface than they 







NATURAL PHILOSOPHY. 


really are, and Indians, who spear them, always try 
to strike perpendicularly, or else aim lower than 
they apparently lie. Water is always deeper than 
it appears. Look obliquely and steadily into a pail 
of water, then place your finger on the outside 
where the bottom seems to be; you will be sur¬ 
prised, on examination, to find that the real bottom 
is several inches below your finger. Fill a glass 
dish with water, and, darkening the windows, let a 
single sunbeam fall upon the surface. The ray will 
be seen to bend as it enters. A little chalk-dust 
scattered through the air will make the beam very 

distinct. Place a 
cent at the bottom 
of a bowl. Standing 
where you cannot 
see the coin, let an¬ 
other person pour 
water into the ves¬ 
sel, when the coin 
will be apparently 
lifted into view. Let L, Fig. 142, be a body be¬ 
neath the water. A ray, L A, coming to the surface, 


Fig. 141. 



Fig. 142. 



is bent downward toward C, and strikes 
the eye as if it came from L'. The ob¬ 
ject will therefore be elevated above 
its true place. In order to understand 
the apparent change of position pro¬ 
duced by refraction, we have but to re¬ 
member this principle —the object is 







OPTICS. 


I 99 


always seen in the direction of the refracted ray as it 
enters the eye. 

Laws of Refraction. —1. In passing into a rarer 
medium, the raj is bent from the perpendicular. 2. 
In passing into a denser medium, the raj is bent 
toward the perpendicular. 

Path of rays through a window-glass. —When a 
raj enters a window-glass, it is refracted toward the 
perpendicular (2d law), and on 
leaving, is equallj refracted 
from the perpendicular (1st 
law). The general direction 
of objects is therefore not 
changed. A poor qualitj of 
glass, however, produces dis¬ 
tortion bj its unequal densitj 
and uneven surface. 0 / B 

Path of rays through a 
prism. —A raj of light, on entering and leaving a 
prism, is re- Fig. 144. 

fracted as in 
the case of a 
window-glass. 

The inclina¬ 
tion of the 
sides, howev¬ 
er, causes the 
raj to be bent 
twice in the same direction. The candle L will there¬ 
fore appear to be at r. 








200 


natural philosophy. 


Lenses—A lens is a transparent body, with at 
least one curved surface. There are two general 
“classes of lenses, concave and convex. Six varieties 
of these are used in optical instruments, viz., the 




double-convex (M), the plano-convex (N), the me¬ 
niscus (O), the double-concave (P), the plano-con¬ 
cave (Q), and the concavo-convex (R). 

The double-convex lens has two convex surfaces, 
ays of light falling upon a convex lens, as upon a 
concave mirror, tend to converge to a focus. A ray 


Fig. 146. 



passing along the line X (the axis of the lens), as it 
strikes the surface perpendicularly, is not refracted. 
The parallel rays M, L, etc., are refracted both on 
entering and on leaving the lens, and are brought 
together at F, the focus. If a light were placed 
at F, its rays would be refracted in parallel lines. 







OPTICS. 


201 


The convex lens is sometimes termed a burning- 
glass. It is used, like the concave mirror, for 
collecting the sun’s rays, and hence is a ready 
means of obtaining fire. Lenses have been manu¬ 
factured of sufficient power to fuse the metals. 
One, of two feet in diameter, made at Leipsic in 1691, 
melted plate-iron, and converted a piece of burnt 
brick into yellow glass. The image formed by a con¬ 
vex lens is, in size and position, precisely like that 

Fig. 147. 



seen in a concave mirror. If we hold a lens above 
a printed page, when we obtain the focal distance 
correctly, we shall find the letters right-side up and 
highly magnified. By an inspection of the figure we 
see that the converging power of the lens simply in¬ 
creases the visual angle, and thus makes the object 
A B appear the size a b. Moving the lens back from 
the page, the letters disappear entirely as we pass 
the principal focus. At length they suddenly reap¬ 
pear again, but smaller and inverted. By examining 
Fig. 148, we see how the rays from A B cross each 
other at the focus, and thus invert the image a b, at 
the same time reducing the visual angle. 

9* 




202 


NATURAL PHILOSOPHY. 


Fig. 148. 



The double-concave lens has two concave sur¬ 
faces. Rays of light falling upon a double concave 
lens, as upon a convex mirror, are scattered. Thus, 


Fig. 149. 



diverging rays from a light at L are rendered more 
diverging; and, to an eye which receives the rays 
M N, the candle would be located at l , where the 
image would be seen. 

The image formed hy a concave lens is, in size and 
position, like that seen in a convex mirror. The 
visual angle is decreased, and the rays do not cross; 
hence the image of A B is seen at a h, erect, and 
diminished in size (Fig. 150). 

Mirage is an optical delusion whereby pictures of 
distant objects are seen as if near. On the heated 
deserts of Africa, the traveller beholds quiet lakes 
and shadows of trees in their cool waters. Rushing 






OPTICS. 


203 


Fig. 150. 



forward to slake his eager thirst, he finds only the 
barren waste of sand. The mariner often sees in 
the sky the images of ships, and the far-distant 
coast, with its familiar cliffs and shipping, so perfect 
in outline as to be instantly recognized. 

The cause is found in the reflection and refraction 
of the rays of light as they traverse layers of air of 
unequal density. In Fig. 151, rays of light from a 
clump of trees at the left are reflected from a layer 
at a, and enter the eye of the Arab as if they came 
from the surface of a lake below. The sandy desert 
itself, shimmering in the hot sun, looks in the dis¬ 
tance not unlike the surface of tranquil water. 
Sometimes, at sea, a layer of air high up in the sky 
acts as a total reflector, and sends down an inverted 
image of ships which are far beyond the horizon. 






204 ' natural philosophy. 

Fig. 151. 



The Composition of Light. 

Solar Spectrum.— When a sunbeam is allowed 
to shine through a prism, the raj is not only bent 
from its course, as we have already seen, but is 
spread out, fan-like, into a broad band of rain¬ 
bow-colors, called the solar spectrum . It contains 
the seven primary colors—Violet, Indigo, Blue, 
Green, Yellow, Orange, Bed. (These may be re¬ 
membered in their order, by noticing that the initial 
letters spell the absurdly meaningless word, Yib-gy- 
or.) If we receive the spectrum on a concave mir¬ 
ror, or pass it through a double-convex lens, it will 
form a white spot. We therefore conclude that 
white light is composed of seven different colors.* 


* Many hold, with Sir David Brewster, that there are hut three 
primary colors— red, yellow, and blue. It is often convenient for 
purposes of explanation to thus consider it. Helmholtz denies the 








OPTICS. 


205 


They are separated because the prism bends them 
unequally. The violet is most refracted and the red 


Fig. 152 







least. The dispersive power of a prism, i. e., its abil¬ 
ity to spread apart the colored rays, depends on the 
material of which it is made. A flint-glass prism is 


truth of this in toto, although he admits that all colors can he 
produced from the three. On the other hand, John Herschel 
claims to have discovered an eighth color below the red, which 
is of a crimson hue, and a ninth beyond the violet, which is of a 
lavender hue. Professor Stokes in addition believes in a tenth 
color beyond the lavender, which he styles the fluorescent ray, as 
it resembles the shade of some kinds of fluor spar. 















206 


NA TURAL PHIL OSOPH T. 


commonly used. A hollow one, filled with oil of 
cassia or bisulphide of carbon, has far higher dis¬ 
persive power. 

Three Classes of Rays in the Solar Spectrum.— 
'These are the calorific , or heat-rays ; the colorific , or 
luminous rays; and the actinic , or chemical rays. 
If we examine the spectrum with a delicate ther¬ 
mometer, we shall find that the heat increases grad¬ 
ually from the violet toward the red, and becomes 
the greatest in the dark space just beyond. If we 
test with a paper containing nitrate of silver, it will 
blacken least in the red, more toward the violet, and 
most in the dark space beyond. Artificial lights 
differ in the proportion of the three classes of rays. 
Seeds will sprout best under a blue glass. Red is 
the warmest color. A photograph could be taken in 
the dark by means of the chemical rays alone. A 
soldier dressed in gray or green clothing is less lia¬ 
ble to be shot than one in red or yellow. 

Complementary Colors. —Two colors, which by 
their mixture produce white light, are termed 
complementary to each other. Let us suppose, 
for simplicity of statement, that white light is 
composed of the three colors, red, yellow, and blue. 
Then, since yellow and blue, when mixed, form 
green, we have red and green as complementary 
colors. Red and blue produce violet; hence yel¬ 
low and violet are complementary colors. If we 
look steadily at a colored wafer lying on a sheet of 
white paper, we shall see a fringe of the comple- 


OPTICS. 


207 

mentarj color play- Fig. 153. 

ing about it. If we 
watch bright red 
clouds, the patches 
of clear sky will seem 
green. In examining 
ribbons of the same 
color, the eye be¬ 
comes wearied and 
unable to detect the 
shade, because of the 
mingling of the complementary color. A knowledge 
of this subject is very essential to the harmony of 
colors in painting, or the arrangement of a bouquet, 
that the result may be pleasing to a cultivated taste. 

Interference of Light. — Newton's Rings. — The 
convex side of a plano-convex lens is pressed down 
upon a flat surface of glass. 

The two surfaces will touch 
each other at the centre; and 
if different circles be described around this point, 
at all parts of each circle the two surfaces will be 
the same distance apart, and the larger the circle 
the greater will be the distance. Now let a beam 
of red light fall upon the flat surface. A black 
spot is seen at the centre; around this a circle 
of red light, then a dark ring, then another circle 
of red light, and so alternating to the circumfer¬ 
ence. By careful measurement it is found that 
the distances between the surfaces of the glass 


Fig. 164. 











208 


NATURAL PHILOSOPHY. 


where the circles of red light appear, are as the 
numbers 1, 2, 3, &c. This, taken in connection 
with what we know already of the theory of wave- 
motion, suggests at once the cause. There are two 
sets of waves, one reflected from the upper surface 
of the plane glass, and the other from the lower 
surface of the convex glass. Where their distance 
apart is less than a wave-length, they interfere and 
produce darkness. Where it is 1, 2, 3, or some 
whole-number of wave-lengths, they coalesce and 
produce a wave of greater intensity. To determine 
the length of a wave of red light, we have only to 
measure the distance between the two glasses at 
the first ring. 

When beams of light of the various colors are 
used, circles of a corresponding color are obtained, 
and, singularly enough, the circles are of dif¬ 
ferent diameters; red light gives the largest, and 
violet the smallest. We hence conclude that red 
waves are the longest, and violet the shortest. 

Length of the Waves .—The minuteness of these 
waves passes comprehension. 40,000 red waves 
and 60,000 violet ones are comprised within a single 
inch. Knowing that light moves at the rate of 
183,000 miles per second, we can easily calculate 
the number of these tiny waves which reach our 
eye in that time. When we look at a red object, 
414 million million of ether waves break on the 
retina every moment, and with a violet color the 
number reaches 666 million million! 


OPTICS. 


209 


Color is exactly analogous to pitch. Violet cor¬ 
responds to the high and red to the low sounds in 
music. Intensity of color, like that of sound, de¬ 
pends on the amplitude of the vibrations. When a 
body absorbs all the colors of the spectrum except 
blue, but reflects that to the eye, we call it a blue 
% body; when it absorbs all but green, we call it a 
green bod} 7 . Red glass has the power of absorb¬ 
ing all except the red rays, which it transmits. 
When a substance reflects all the colors to the eye, 
it seems to us white. If it absorbs all the colors, it 
is black. A tint is produced by a mingling of waves 
of different colors. We thus see that color is not an 
inherent property of the objects around us. In the 
darkness all bodies are devoid of color. 

The play of colors seen in mother-of-pearl is due 
to the interference of light in the fine grooves caused 
by the edges of the thin overlapping plates of which 
it is composed. The same effect may be seen in a 
putty mould of the pearl. In a similar manner the 
plumage of certain birds reflects changeable hues. 
A metallic surface ruled with fine parallel lines not 
more than ^Vir of an inch apart, gleams with bril¬ 
liant colors. Thin cracks in plates of glass or quartz, 
mica, when two layers are slightly separated—even 
the scum floating in stagnant water, break up the 
white light of the sunbeam and reflect the varying 
tints of the rainbow. The rich coloring of a soap- 
bubble is given it by the film of water which runs 
from the top down the sides, and thus produces in¬ 
terference of light. 



210 


NATURAL PHILOSOPHY. 


Diffraction of light is caused by a beam of light 
passing along the edge of some opaque body. As 
the waves of ether strike against it, they put in 
motion another set of waves on the opposite side 
which interfere with the first system. If we hold a 
fine needle close to one eye and look toward the 
window, we shall see several needles. Place the 
blades of two knives closely together and hold them 
up to the sky; a most beautiful set of waving fines 
of interference will shade the open space. Most 
delicate colors are seen by looking at the sky through 
the meshes of a veil, or at a lamp-light through a 
bird-feather or a fine slit in a card. 

Polarized Light. —Double Refraction. — If we could 
Fig. 155. look at the end of a ray of light as we 

® can at the end of a rod, we should see 
the particles of ether swinging swiftly 
to and fro, crosswise, in the direction of 
all the diameters (Fig. 155). Certain 
crystals have the power of sifting and arranging 
these vibrations into two sets at right angles to each 


Fig. 156. 



other, thus making a ray of the form seen in Fig. 
156. As one set is more refracted than the other in 
passing through the crystal, the ray is divided into 
two rays—the ordinary and extraordinary. Hays 



OPTICS. 


211 


which have been thus sifted constitute polarized 
light. Iceland spar (Fig. 157) possesses this property 
of double refraction in a re¬ 
markable degree. An object 
viewed through it in any 
direction not parallel to 
a b (the optic axis) appears 
double. If the crystal is 
placed on a dot and slowly turned around, two dots 
will be seen, the second revolving about the first. 
Light may be po- Flg m 

larized by reflection 
from glass at a 
fixed angle, and also 
by passing through a 
thin slice of tourma¬ 
line ,— a transparent 
crystal which ab- 

. . n . Object seen through Iceland Spar. 

sorbs the ordinary 

and transmits the extraordinary rays. Objects ex¬ 
amined by means of polarized light present many 
curious changes. A crystal of quartz or mica, which 
appears to the eye like common glass, reveals a se¬ 
ries of beautifully colored rays, due to the inter¬ 
ference of the ordinary and extraordinary rays of 
light. If we look at a lamp-light through a piece of 
common isinglass, we shall see a beautiful series of 
polarized rays having a star-like form. The angle 
of the rays varies with different kinds of mica. If 
polarized light be passed through common glass no 



Fig. 151. 






2 12 


NATURAL PIHLOSOPEY. 


change is seen, but on slight pressure a system of 
variegated colors appears. This method of exami¬ 
nation presents a most delicate means of determin¬ 
ing the molecular structure of a body. Some sub¬ 
stances have the power of twisting the plane of the 
polarized light. Grape-sugar turns it to the right, 
and fruit-sugar to the left. The French Govern¬ 
ment use a polarizing instrument, in which this prin¬ 
ciple is applied to test the quality of the sugar im< 
ported into France. 

The Rainbow is formed by the refraction and re- 
flection of the sunbeam in drops of falling water. 
The white light of the sun is thus decomposed into 
its simple colors. The inner arch is termed the 
primary bow; the outer or fainter arch, the second¬ 
ary. Each of these contains all the colors of the 
spectrum, but in reverse order. The rainbow is al¬ 
ways seen in the quarter of the heavens opposite to 
the sun. 

Primary Bow .—A ray of light, S", enters, and is 
bent downward at the top of a falling drop, passes 
to the opposite side, is there reflected, then passing 
out of the lower side, is bent upward. ~By the re¬ 
fraction the ray of white light is decomposed, so 
that when it emerges it is spread out fan-like, as in 
the solar spectrum. Suppose that the eye of a spec¬ 
tator is in a proper position to receive the red ray, 
he cannot receive any other color from the same 
drop, because the red is bent upward the least, and 
all the others will pass directly over his head. He 


OPTICS. 


213 

sees the violet in a drop below. Intermediate drops 
furnish the other colors of the spectrum. 


Fig. 159. 



Secondary Bow .—A ray of light, S, strikes the 
bottom of a drop, v, is refracted upward, passes to 
the opposite side, where it is twice reflected, and 
thence passes out at the upper side of the drop. 
The violet ray being most refracted, is bent down 
to the eye of the spectator. Another drop, r, re¬ 
fracting another ray of light, is in the right position 
to send the red ray to the eye. 

Why the Boiu is circular .—In the primary bow it 
is found that when the red ray leaves the drop, it 
forms an angle with the sun’s ray, S r, of about 42°, 
the violet 40°. These angles are constant. Let a h 
be a straight line drawn from the sun through the 
observer’s eye. If produced, it would pass through 


214 


NATURAL PHILOSOPHY. 


the centre of the circle of which the rainbow is an 
arc. This line is termed the visual axis. It is par¬ 
allel to the rays of the sun; and when it is also 
parallel to the horizon, the rainbow is an exact semi¬ 
circle. Suppose the line E v in the primary bow to 
be revolved around E b, keeping the angle b~Ev un¬ 
changed ; the point v would describe a circle on the 
sky, and every drop over which it would pass would 
be at the proper angle to send a violet ray to the eye 
at E. Imagine the same with the drop r. We can 
thus see (1) that the bow must be circular; (2) that 
when the sun is high in the heavens, the whole bow 
sinks below the horizon; (3) that the lower the sun 
the larger is the visible circumference; and (4) that 
on lofty mountains a perfect circle may sometimes 
be seen. 

Halos, coronas, sundogs, circles about the moon, 
the gorgeous tinting at sunrise and sunset, are 
all produced by the refraction and reflection of the 
sun’s rays when passing through the clouds in the 
upper regions of the atmosphere. The phenom¬ 
enon familiarly known as the “ sun’s drawing wa¬ 
ter,” consists merely of the long shadows of broken 
clouds.* 

Spherical Aberration. —Kays which pass through 
a lens near the edge are brought to a focus sooner 
than those nearer the centre. Therefore, when an 

* Spectrum analysis, twilight, and other kindred topics, are 
best understood in their relations to Astronomy. See “ Fourteen 
W eeks in Astronomy.” 



OPTICS . 


2I 5 


image is clear around the edge, it will be indistinct 
at the centre, and vice versa. This wandering of the 
rays from the focus is termed spherical aberration. 

Chromatic aberration is caused by the different re- 
frangibility of the several colors which compose 
white light. The violet, being bent most, tends to 
come to a focus sooner than the red, which is bent 
least. This causes the play of colors seen around 
the image produced by an ordinary glass. It is 
Fi &- 160 - remedied by using a second 

lens of different dispersive 
power, which counteracts the 
effect of the first. (Fig. 160.) 
Such a lens is said to be achromatic (colorless). 

Optical Instruments. 

Microscopes (to see small things) are of two kinds, 
simple and compound. The former consists of a 
double convex lens; the latter contains at least 
two lenses. 

At M is a mirror which reflects the rays of 
light through the object a. The object-lens o 
forms, in the tube above, a magnified inverted 
image of the object. The eye-lens 0 magnifies this 
image. If a microscope increases the diameter of 
an object 100 times, it is said to have a power of 
100 diameters. In that case the surface is magnified 
100 2 =10,000 times. To prevent spherical aberra¬ 
tion the object-lens is made very small. 





216 


NATURAL PHILOSOPHY. 



Fig. 161. 


A Compound Microscope. 

Telescopes {to see afar off) are of two kinds, re* 
fleeting and refracting . The former contains a large 
metallic mirror (speculum) which reflects the rays 
of light to a focus. The observer stands at the 
side and examines the image with an eye-piece. 









OPTICS 



21 7 


Tier. lr>2. 


A Reflecting Telescope. 

The largest reflecting telescope ever made is that of 
Lord Kosse. Its speculum has a diameter of 6 feet, 
and a focal distance of 53 feet. (See frontispiece of 
Astronomy.) 

The refracting telescope, like the microscope, con- 
10 
















































2l8 


NATURAL PHILOSOPHY. 


tains an object-lens o which forms an image a b . 
This is viewed by means of the eye-piece 0, which pro¬ 
duces a magnified and inverted image cd. The objects 
seen in the heavens are so far distant that the rays 
of light are nearly parallel, and hence there is little 
spherical aberration. The object-lens may there* 


Fig. 163. 



fore be made of any size without rendering the im¬ 
age indistinct. The larger this lens, the more light 
is collected with which to view the image. The 
magnifying power is principally due to the eye¬ 
piece. The great telescope in the observatory at 
Chicago is the best in the world. The diameter of 
its object-glass is 18^ inches—equivalent to enlarg¬ 
ing the pupil of the astronomer’s eye to that size. 
It was made by Alvan Clark & Sons, of Cambridge, 
Massachusetts. 

The inversion of the object is of no practical im¬ 
portance for astronomical purposes. For terrestrial 
observations additional lenses are used to invert the 
image. 

The opera-glass contains an object-glass 0 and an 
eye-piece o. The latter is a double concave lens; 
this increases the visual angle by diverging the rays 






OPTICS. 


219 


Fig. 164. 

A 


it 

of light, which would otherwise come to a focus be¬ 
yond the eye-piece. An erect and magnified image 
is seen at a b. 

The stereoscope contains por¬ 
tions of two convex lenses, as 
shown in Fig. 165. Two photo¬ 
graphs A and B are taken by two 
cameras which are slightly in¬ 
clined to each other. This pro¬ 
duces two pictures precisely 
like the two views we always 
obtain of an object by the use 
of both eyes. The blending of 
the two at C causes the appear¬ 
ance of solidity. 

The magic lantern , or stereop- 
ticon, contains a reflector M, 
which condenses/ the rays of a powerful oil or cal¬ 
cium light upon a lens L. They are here converged 
upon the object ab. Thence a double lens m throws 
a highly magnified image on the screen A B. Dis¬ 
solving views are produced by the use of two lanterns 
which contain the separate scenes which are to melt 
into each other. 


Fig. 165. 



















220 


NA TURAL PHIL 0S0PHY. 


Fig. 166. 



The Camera used by photographers contains a 
double-convex lens at A, which throws an inverted 


image of the object upon the ground-glass screen 

EB. 

The Eye is the most perfect optical instrument. 
It is rarely, if ever, troubled by spherical or chro¬ 
matic aberration, and is self-focusing. It closely 
resembles a camera. The outer membrane of the 
eye is termed the Sclerotic coat, S. It is tough. 



Fig. 167. 














OPTICS. 


22 1 


white, opaque, and firm. A little portion in front, 
termed the Cornea, c, is transparent; this is convex, 
and is set into the sclerotic coat somewhat like a 
watch-crystal. The middle or Choroid coat, C, is soft 

Fig. 168. 



A 



and delicate, like velvet. It lines the inner part of 
the eye. It is covered with a black pigment, which 
absorbs the superfluous light. Upon it the optic 
nerve, which enters at the rear, expands in a net¬ 
work of delicate fibres termed the Retina. This 
is the seat of vision. Back of the cornea is a trans¬ 
parent limpid fluid, the aqueous humor. The anterior 
chamber, filled with this liquid, is closed at the back 
by a colored curtain, h i, the Iris. The Pupil is a 
round hole in the Iris. The Crystalline lens , o, sep¬ 
arates the two chambers of the eye. It is a double- 
convex lens, tough as gristle and transparent as 
glass. It is composed of concentric layers, like an 
onion. The posterior chamber is filled with the 
vitreous humor , which is a transparent, jelly-like 
liquid, resembling the white of an egg. 

Let A B represent an object in front of the eye. 





222 


NA TURA L rillLOSOPIIY. 


Bays of light are first refracted by the aqueous 
humor, then falling upon the crystalline lens they 
are further refracted, and lastly are refracted by the 
vitreous humor and form an image a b on the retina. 
This is smaller than the object, and inverted. 

The more distant the object, the smaller the pic¬ 
ture. To render vision distinct, the rays must be 
accurately focused on the retina. If we gaze stead¬ 
ily at an object near by, and then suddenly observe 
some distant one, we find our vision blurred. In a 
few moments it becomes clear again. This shows 
that the eye has the power of adapting itself to the 
varying distances of objects, which is done by a vari¬ 
ation in the convexity of the crystalline lens. 

When a body is held very near the eye, the lens 
has not sufficient power to converge the rays upon 
the retina in a distinct image. When the distance 
at which a clear vision takes place is less than four 
or five inches, the person is said to be near-siglited , 
and when greater than ten or twelve, to be far¬ 
sighted. This difference lies in the shape of the eye¬ 
ball. In far-sightedness the ball is too flat, and the 
retina is too near the lens ; in near-sightedness the 
ball is elongated, so that the retina is too far back 
from the lens. The former can be remedied by con¬ 
vex glasses, which bring the rays to a focus sooner, 
and the latter by concave glasses, which throw the 
focus further back. In old age the eye loses the 
power of adjusting the crystalline lens; elderly 
people, therefore, hold a book at some distance 


OPTICS. 


223 


from the eye. They are aided by using convex 

glasses. 

The retina retains any impression made upon it 
for about one-eighth of a second. This explains 
why a wheel, when rapidly revolved, appears solid, 
or a lighted brand like a ring of fire. On the other 
hand, it requires a moment for an impression to be 
made. Thus a wheel may be whirled so swiftly that 
its spokes become invisible. 

Some eyes are entirely uninfluenced by certain 
colors, as some ears are deaf to certain sounds. 
Color-blindness is most commonly noticed in refer¬ 
ence to red, green, and blue. Doubtless railway ac¬ 
cidents have often occurred through this natural in¬ 
ability to distinguish signals. Dr. Mitchell mentions 
the case of a naval officer who chose for his uni¬ 
form a blue coat and red waistcoat, fully believing 
them to be of the same color. He also tells of a 
tailor who mended a black silk waistcoat with a 
piece of crimson, and of another who put a red col¬ 
lar on a blue coat. Dalton could only see two colors, 
blue and yellow, in the solar spectrum, and having 
once dropped a piece of red sealing-wax in the 
grass, he could not find it by the difference in color. 
The range of the eye is much less than that of the 
ear. The latter is about eleven octaves, while the 
former never exceeds a single octave. 

The diameter of the eye is less than an inch; 
yet, as we look over an extended landscape, every 
feature, with all its variety of shade and color, 


224 


NATURAL PHILOSOPHY. 


is repeated in miniature on the retina. Millions 
upon millions of ether waves, converging from every 
direction, break on that tiny beach, while we, obliv¬ 
ious to all the marvellous nature of the act, think 
only of the beauty of the revelation. 

\ 'Practical Questions. —1. Why is the secondary how fainter than the 
primary? Why are the colors reversed? 2. Why can we not see around the 

* corner of a house, or through a bent tube? 3. What color would a painter 
use if he wished to represent an opening into a dark cellar? 4. Is white a 
color? Is black? 5. By holding an object nearer a light, will it increase or 
diminish the size of the shadow ? 6. What must be the size of a glass in or¬ 
der to reflect a full-length image of a person ? Ans. Half the person’s height. 
7. Where may we see a rainbow in the morning ? 8. Can two spectators see 
the same bow? 9. Why, when the drops < f water are falling through the air, 
does the rainbow appear stationary? 10. Why can a cat see in the night? 
11. Why cannot an owl see in daylight ? 12. Why are we blinded when we 

pass quickly from a dark into a brilliantly lighted room ? 13. If the light of 
the sun upon a distant planet is only Vioo of that which we receive, how does 
its distance from the sun compare with ours ? 14. If when I sit six feet from 
a candle I receive a certain amount of light, how much shall I diminish it if I 
move back six feet furt her ? 15. Why do drops of rain, in felling, appear like 

liquid threads? 16. Why does a towel turn darker when wet? 17. Does 
color exist in the object, or in the mind of the observer ? 18. Why is lather 
opaque, while air and a solution of soap are each transparent? 19. Why 
does it whiten molasses candy to “pull it?” 20. Why does plastering be¬ 
come lighter in color as it dries? 21. Why does the photographer use a 
kerosene oil lamp in the “dark room?” 22. Is the common division of 
colors into “cold” and “warm” verified in philosophy? 23. Why is the 
image on the camera, Fig. 167, inverted? 24. Why is the second image seen 
in a mirror, Fig. 134, brighter than the first? 25. Why does a blow on the 
head make one “ see stars ? ” Ans. The blow excites the optic nerve, and so 
produces the sensation of light. 26. What is the principle of the kaleido¬ 
scope ? Ans. It contains three mirrors set at an angle of 60°. Small bits of 
colored glass at one end reflect to the eye, at the other, multiple images which 
change in varying patterns as the tube is revolved. 27. Which can be heard 
at the greater distance, noise or music ? 



HEAT. 



Definitions. —Luminous heat is that which radi¬ 
ates from a luminous body. Ex.: An iron heated 
to whiteness. Obscure heat is that from a non-lumi- 
nous source. A diathermanous (diet, through, and 
thermos, warm) body is one which allows the heat to 
pass through it readily. Kock-salt is the most per¬ 
fect diathermanous solid known. It is to heat what 
glass is to light. Cold is a merely relative term, in¬ 
dicating the absence of heat in a greater or less de¬ 
gree. Gases and Vapors differ but slightly. The 
former retain their form at all ordinary tempera¬ 
tures ; Ex.: Air. The latter are readily condensed ; 
Ex. : Steam. 

The intimate relation between Light and Heat. 
—Thrust a cold iron into the fire. It is at first dark, 
but soon becomes luminous, like the glowing coals. 
Raise the temperature of a platinum wire. We soon 
feel the radiation of obscure heat-rays. As the 
metal begins to glow, our eye detects a red color, 
then orange combined with it, then green, and so 
on through the scale of the spectrum. At last all 



228 


NATURAL PHILOSOPHY. 


the colors are emitted, and the metal is dazzling 
white. All bodies become luminous at a fixed tem¬ 
perature. Heat may be reflected, refracted, and 
even polarized. It radiates in straight lines equally 
in every direction, and decreases in intensity as the 
square of the distance. It moves with the same 
velocity as light. Heat and light come to us com¬ 
bined in the sunbeam. It is therefore believed that 
they are the same—that light is only luminous heat, 
and that the three classes of waves in the solar 
spectrum differ, as one color differs from another, 
in the rapidity of the vibrations. The longer and 
slower waves of ether fall upon the nerves of touch, 
and produce the sensation of heat.* The more rapid 
ones are peculiarly adapted to affect the optic nerve 
and produce the sensation of light. The shortest 
and quickest cause chemical changes. 

Theory of Heat.— Heat is only a mode of motion. 
Rest is unknown in nature. Even the molecules of 


* It is now believed that the particles of the nerves vibrate, 
and thus communicate to the brain the impressions made by ex¬ 
ternal objects. Each of the five classes of nerves seems to be 
adapted to transmit vibrations of its own kind, while it is insen¬ 
sible to all others. Thus, if the rate of oscillation be less than 
that of red, or more than that of violet, the optic nerve is unin¬ 
fluenced by the waves. We cannot see with our fingers, nor 
taste with our ears. Nerves transmit motion at the rate of about 
93 feet per second. If, then, a man, six feet high, were to step on 
a nail, it would require nearly an eighth of a second for the in¬ 
formation to be carried by the sensor nerve to the brain, and for 
the order to lift the foot to be returned by the motor nerve to the 
suffering member. 



heat .. 


229 

a solid are in constant vibration. As the worlds in 
space are revolving about each other in inconceiv¬ 
ably vast orbits, so each body forms a miniature 
system, its molecules revolving in inconceivably 
minute orbits. When we increase the rapidity of this 
motion, we heat the body; when we decrease it, we 
cool the body. The vacant spaces between the 
molecules are filled with ether. As the air moving 
among the limbs of a tree sets its boughs in motion, 
and in turn may be kept in motion by the waving 
branches, so this ether may put the molecules in 
vibration, or be thrown into motion by them. Ex.: 
Insert one end of a poker into the fire. The parti¬ 
cles in contact with the heat are made to vibrate 
intensely; these swinging atoms strike their neigh¬ 
bors, and so on, atom by atom, until the oscillation 
reaches the other end. If, now, we handle the poker, 
the motion is imparted to the delicate nerves of 
touch ; they carry it to the brain, and pain is felt. 
In popular language, “ the iron is hot,” and we are 
burned. If, without touching it, we hold our hand 
near the poker, the ether waves set in motion by the 
whirling atoms of iron strike against the hand, and 
produce a less intense sensation of heat. In the 
former case, the fierce motion is imparted directly; 
in the latter, the ether acts as a carrier to bring it 
to us. 

Quality of Heat. —As some sounds are shrill and 
piercing, others deep and heavy, so some kinds of 
heat are keen and penetrating, others mild and dif- 


23° 


NATURAL PHILOSOPHY. 


fusive. This difference depends on the length of the 
waves and their combined rates of vibration. Thus 
the chirp of the cricket compares with the heat of a 
glowing furnace, and the soft tones of an organ with 
the genial radiation of a steam-pipe. Pitch in mu* 
sic, variety in color, and degrees in heat, are there¬ 
fore intimately related. 

\The Sources of Heat are the sun, stars, mechan¬ 
ical and chemical forces. 

(1.) The molecules of the sun and stars are in 
rapid vibration. These set in motion waves of 
ether, which dart with the velocity of light across 
the intervening space, and surging against the 
earth, give up their motion to it. (2.) Friction 
and percussion produce heat, because additional 
motion is thereby imparted to the vibrating par¬ 
ticles. Savages obtain fire by rubbing together 
two pieces of wood. A horse hits his steel shoes 
against a stone and “ strikes firelittle particles of 
the metal torn off are heated by the shock, so that 
they burn as sparks. The bearings of machinery 
become hot, unless the friction is diminished by 
grease. A train of cars is stopped by the pressure 
of the brakes. If we watch them in a dark night, 
we shall see the sparks flying from the wheels^ the 
motion of the train being converted into heat, A 
blacksmith pounds a piece of iron until it glows. 
The force of his strokes sets the particles of the 
metal vibrating rapidly enough to send ether waves 
of such swiftness as to affect the eye of the ob- 


HEAT. 


231 


server.* A piece of wood may be heated by simply 
squeezing it in a hydraulic press. At the exposition 
in Paris, chocolate was kept hot by means of two 
copper plates which were rubbed together by ma¬ 
chinery. As a cannon-shot strikes an iron target, a 
sheet of flame pours from it. Our earth moves with 
a velocity of over G8,000 miles per hour. Were it 
instantly stopped, enough heat would be produced 
to change the entire globe to vapor. (3.) Chemical 
action is most commonly seen in fire. The oxygen 
of the air has an affinity for the carbon and hydro¬ 
gen of the fuel. They rush together. As they strike, 
their motion is stopped. The shock sets the mole¬ 
cules in vibration. They impart their motion to the 
ether, and thus start waves of heat. 

The Mechanical Equivalent of Heat.— In tlieie 
various changes of mechanical-motion into lieat- 
motion no force has been lost. The blacksmith’s 
hammer falling on the anvil gives rise to a definite 
amount of heat. If the heat could be gathered up, 
it would be sufficient to lift the hammer to the point 
from which it fell. No force can be annihilated. 
If destroyed in one form, it reappears in another 
without loss. A 'pound-weight foiling from a height 
of 11% feet, would generate enough heat to raise the 


* Text-books frequently assert that “ iron, once treated in this 
way, cannot again be made red-hot by hammering, unless subse-* 
quently reheated in the forge.” Even Miller gives currency to 
this statement. Any blacksmith will convince you of its utter 
falsity by actual experiment. 



23 2 


NATURAL PHILOSOPHY. 


temperature of one jpound of water one degree; con¬ 
versely, the amount of heat necessary to elevate 
a pound of water one degree, would raise a pound- 
weight to the height of 772 feet. This is called 
“ Joule's laiof or the “ mechanical equivalent of heat.” 


Change of State by Heat. 

Latent, Sensible, and Specific Heat. —When a 
body is heated, the heat-force is divided into two 
parts: one portion elevates the temperature, and 
the other increases the size. The former can be 
detected by the touch, and is called sensible heat. 
The latter tends to counteract the force of Cohesion, 
and is neutralized so that it cannot be detected by 
the touch; it is therefore termed latent heat. Sub¬ 
stances vary in their application of the heat-force. 
Some devote more to temperature, others to expan¬ 
sion. Therefore, if the same amount of heat be ap¬ 
plied to the same bulk of different substances, they 
will not indicate the same temperature ; and on the 
other hand, when various bodies indicate the same 
temperature by a thermometer, they may possess 
vastly different quantities of heat. Steam contains 
the greatest amount of latent heat of any known sub¬ 
stance, except hydrogen, yet it indicates no higher 
temperature than boiling water. The relation be¬ 
tween sensible and latent heat is termed specific 
heat. It is the quantity of heat that is required to 
raise a given weight of any substance 1° in tempera¬ 
ture, compared with the quantity required to elevate 


HE A T. 


233 


the same weight of water 1°. Thus, a quantity of 
heat which elevates the temperature of a pound of 
water 1° would raise that of a pound of mercury 30°. 
Hence, taking water as the standard, the specific 
heat of mercury is only .033. 

jLatent heat is not lost .—In the various changes of 
state of which we shall now speak, wherein bodies 
pass from solid to liquid and from liquid to the 
gaseous form, sensible heat becomes latent. Thus, 
one who has melted snow, or “ boiled away” water, 
knows how slow is the process, and how much heat 
is consumed. When the vapor or liquid passes back 
into its original state, the latent heat is restored 
again as sensible heat. The following curious para¬ 
dox illustrates this thought: Freezing is a warming 
process, and thaioing a cooling process . 

Freezing-mixtures depend on the principle of latent 
heat. Their most common use is in freezing ice¬ 
cream. Salt and powdered ice are employed. Salt 
has a great attraction for water. It therefore dis¬ 
solves the ice to get it, and then itself dissolves in 
the water thus obtained. In this process two solids 
pass into the liquid form. The heat necessary for 
this change of state is absorbed mainly from the 
cream. 

I. Expansion. —By the addition of heat the mole¬ 
cules are urged into swifter motion, and therefore 
pushed further apart, increasing the size of the body. 
Hence the law, “ Heat expands and cold contracts 

(1.) Solids expand uniformly; i. e., a definite rise 


234 


NATURAL PHILOSOPHY. 


in temperature produces a fixed increase of size. 
Different substances, however, expand unequally. 
Zinc dilates more than iron, and iron more than 
glass. The force of the expansion is irresistible. 
It is said that iron, heated from zero up to the boil¬ 
ing point of water, exerts a pressure equal to 16,000 
times that of gravity. When it cools, it contracts 
with the same force. Practical applications of this 
principle abound in the arts. A carriage-tire is put 
on when hot, that, when cooled, it may bind the 
wheel together. Rivets used in fastening the plates 
of steam-boilers are inserted red-hot. “ The pon¬ 
derous iron tubes of the Britannia bridge writhe and 
twist, like a huge serpent, under the varying influence 
of the solar heat. A span of the tube is depressed 
but a quarter of an inch by the heaviest train of 
cars, while the sun lifts it 2J inches.” The Bunker- 
hill monument nods as it follows the sun in its daily 
course. Lead and zinc, on cooling, do not contract 
to their original dimensions, but their particles 
slide over each other in expanding; in this man¬ 
ner the linings of sinks become puckered. On the 
other hand, this force of expansion must be guarded 
against. In laying long water-pipes, some of the 
tubes are made to slip into each other with tele¬ 
scopic slides. Tumblers of thick glass often break 
on the sudden application of heat, because the sur¬ 
face dilates before the motion has time to reach 
the interior. Draughts of cold air frequently crack 
heated lamp-chimneys, for a similar reason. (2). 


HEAT. 


235 


Liquids are much more sensitive to heat than solids, 
but do not expand as equally. . (3). Gases expand 
uniformly jfe of their bulk. 490 cubic inches of 
any gas, at 32° F., if heated 1°, become 491 cubic 
inches. 

The Mercurial Thermometer is an instrument for 
measuring the temperature by the expansion of 
mercury. As this metal freezes at —39° F., colored 
alcohol is used for low temperatures. The method 
of filling a thermometer may be illustrated in the 
following manner. Take the glass tube shown in 
Fig. 67, and hold the bulb in the flame of an alcohol 
lamp until the air is nearly expelled. Then plunge 
the stem in some colored water. As soon as the 
bulb cools, the water will rise and partly fill it. 
Heat the bulb again in the flame until the steam 
pours out of the stem. On inserting it a second 
time, the water will entirely fill the bulb. In the 
manufacture of thermometers, it is customary to have 
a cup blown at the upper end of the stem. This 
is filled with mercury, and the air, when expand¬ 
ed, bubbles out through it, while the metal trickles 
down as the bulb cools. The mercury is now heated 
to as high a temperature as the thermometer is in¬ 
tended to measure, when the tube is melted off and 
sealed at the extremity of the column of mercury. 
The metal contracts on cooling, and leaves a vacuum 
above. Each thermometer is graduated separately. 
It is put in melting ice, and the point to which the 
mercury sinks is marked 32°— Freezing-point. It is 


NATURAL PHILOSOPHY. 


236 

then placed in a steam-bath, and the point to which the 
mercury rises (when the barometric column stands at 
30 in.) is marked 212°— Boiling-point. This constitutes 
what is called Fahrenheit’s scale (F.) 
It is said that the inventor placed the 
zero-point 32° below the temperature of 
freezing water, because he thought that 
to be absolute cold. In the Centigrade 
scale (C.), the freezing-point is marked 0, 
and the boiling-point 100. 1° C. = 1.8°F. 
In Beaumur’s scale (B.), the boiling-point 
is fixed at 80°. 

II. Liquefaction. —When heat is add¬ 
ed to a solid body, the temperature rises 
until the freezing-point (melting-point) is 
reached, when it becomes stationary. 
The force is now all applied to neutral¬ 
izing the Cohesive attraction. The expan¬ 
sion continues. The molecules are pushed further 
and further apart, until, escaping the grasp of Cohe¬ 
sion, they move freely on each, other. This constitutes 
liquefaction, as seen in the melting of ice, iron, &c. In 
this process large quantities of heat become latent in 
the body. If ice at 32° be melted, 142° of heat will 
disappear, and the water will be at only 32°. Hence, 
to convert 1 lb. of ice at 32° into water at 32°, 
enough heat must be used to raise 142 lbs. of water 
from 32° to 33°. 

Liquefaction of gases .—When a gas is cooled the 
repellant force is weakened, and the molecules once 










HEAT . 


237 



The vapor thus formed does not contain any solid 
which may be dissolved in the liquid. This princi¬ 
ple is applied to distillation. Ex. : Pure or distilled 
water is obtained by heating it in a boiler A, whence 


more approach each other. By the combined action 
of cold and pressure the particles of almost every 
known gas have been brought near enough for the at¬ 
traction of Cohesion to grasp them. When the pres¬ 
sure is removed, the gaseous form is quickly resumed. 

III. Vaporization. —If heat be applied to a liquid, 
the temperature rises until the boiling-point is reached, 
when it stops. The expansion, however, continues 
until the motion is so violent as to overcome the Co¬ 
hesive force and to throw off particles of the liquid. 


Fig. 170. 
















NATURAL PHILOSOPHY. 


238 

tlie steam passes through the pipe C and the worm, 
within the condenser S, where it is condensed and 
drops out into the vessel D. The pipe is coiled in a 
spiral form within the condenser, and is hence termed 
the worm. The condenser is kept full of cold water 
by means of the tub at the left. By careful regula¬ 
tion of the heat, one liquid may be separated from 
another by distillation. (See Chemistry, p. 196.) 

The boiling-point .—When we heat water, the bub¬ 
bles which pass off first contain merely the air dis¬ 
solved in the liquid ; next bubbles of steam form on 
the bottom and sides of the vessel, and, rising a 
little distance, are crushed in by the cold water and 
condensed. In breaking they produce that peculiar 
sound known as “ simmering.” They ascend higher 
and higher as the temperature of the water rises, 
until at last they break at the surface, and the steam 
passes off into the air. The violent agitation of the 
water produced by the passage of these steam bub¬ 
bles is termed boiling. The boiling-point is not the 
same in different liquids. This produces the variety 
we see in the forms of matter. Some vaporize at 
ordinary temperatures ; others only melt at the very 
highest; while the gases of the air are but the steam 
of substances which vaporize at enormously low 
“temperatures. The boiling-point of water depends 
on three circumstances. 

(1.) The purity of the loater .—Any substance which 
increases the cohesive power of the water elevates 
the boiling-point. For this reason salt water boils 


HEAT. 


23 9 


at a higher temperature than pure water. The aii 
dissolved in water tends by its elastic force to sepa- 
rate the molecules. If this be removed, the boiling- 
point is elevated as high even as 275°, when the 
water is converted into steam with explosive 
violence. 

(2.) The nature of the vessel .—Water will boil at a 
lower temperature in an iron than in a glass vessel. 
If the surface of the glass be made chemically clean, 
the boiling-point is elevated still higher. This seems 
to depend on the strength of the adhesion between 
the water and the vessel in which it is contained. 

(3.) The pressure .—Any pressure upon the surface 
tends to keep the molecules together, and so raises 
the boiling-point. Water, therefore, boils at a lower 
temperature on a mountain than in a valley. The 
temperature of boiling water at Quito is 90°, and on 
Mont Blanc 84° C. The variation is so uniform, 
that the height of any place can be ascertained with 
tolerable accuracy by this means. A difference of 
1° F. is produced by an ascent of 596 feet. 

The influence of pressure is very finely illustrated 
by the following experiment. Boil a glass flask half 
full of water for some time. Cork it quickly and then 
invert it. The pressure of the accumulated steam 
will soon stop all ebullition. A few drops of cold 
water will condense the steam, and boiling will com¬ 
mence again. This will soon be checked, but can be 
restored as before. The process may be repeated 
until the water cools to little more than blood-heat. 


240 


NA TUBAL PHILOSOPHY. 


Fig. 171. 


If the cork be air¬ 
tight, when the water 
is quite cold it will 
strike with a sharp 
metallic sound as it 
falls from one end of 
the flask to the other. 
The cushion of air 
which commonly 
breaks the fall of wa¬ 
ter is here removed. 
The water-hammer il¬ 
lustrates this point 
yet more fully.- It 
consists of a glass 
tube half full of wa¬ 
ter, from which the 
air has been expelled by heat, the tube being sealed 
while the water was yet boiling. The vacuum is 
very perfect; steam may be produced in it by the 



Fig. 172. 




heat of the hand, and the water falls to and fro with 
the apparent force of lead. The pulse-glass shown 
in the figure is a somewhat similar instrument. 









HE A T. 


24 


The temperature cannot be raised above the boil¬ 
ing-point, unless the steam is confined, however 
much heat may be applied. The extra force is 
entirely occupied in expanding the water into steam. 
This occupies 1,700 times the space, and is of 
the same temperature as the water from which 
it is made. Over 900° of heat become latent in 
this proc»ss, but are made sensible again when 
the steam is condensed. The common method 
of heating by steam depends upon this fact. Steam 
is invisible. This we can verify for ourselves by 
examining it just as it issues from the spout of the 
tea-kettle. It soon condenses, however, into minute 
globules, which, floating in the true steam, render 
the vapor apparently visible. 

IY. Evaporation should be distinguished 
vaporization. It is a slow formation of vapor, which 
takes place at all ordinary temperatures. Ex.: Wa¬ 
ter evaporates slowly, even at the freezing-point. 
Clothes dry in the open air in the coldest weather. 
The wind quickens the process, because it drives 
away the moist air near the clothes and supplies its 
place with dry air. Evaporation is also hastened 
by an increase of surface and a gentle heat. This 
principle is made useful in the arts for separating a 
solid from the liquid which holds it in solution. 

Vacuum pans are largely employed in condensing 
milk, in the manufacture of sugar, etc. They are 
so arranged that the air above the liquid in the ves¬ 
sel may be exhausted, and then the evaporation takes 
11 



242 


NATURAL PHILOSOPHY. 


place very rapidly, and at so low a temperature that 
all danger of burning is avoided. 

Evaporation cools a 
liquid very rapidly, since 
sensible heat becomes la¬ 
tent in the vapor. Pious 
Mahometans were former¬ 
ly accustomed to place, in 
niches along the public 
streets, porous earthen¬ 
ware bottles filled with 
water, to refresh the thirsty 
travellers. Water may even 
be frozen in a vacuum, if the vapor be removed as 
fast as formed. Ice is manufactured in the tropics 
by machines constructed on this principle. The 
greatest artificial cold ever known, —220° F., was 
produced by evaporating in a vacuum a mixture of 
liquid nitrous oxide gas and bisulphide of carbon. 

Spheroidal State.— If a few drops of water be 
put in a red-hot metallic cup, they will gather into 
a globule, which will dart to and fro over the surface 
with little diminution. It seems to rest on a little 
cushion of steam, which supports it while the heated 
currents of air drive it hither and thither. If the 
cup be allowed to cool, after a little the water will 
lose its spheroidal form, and coming into direct 
contact with the metal, burst into steam with a 
slight explosion. This principle may perhaps ac¬ 
count for some unexplained boiler accidents. By 








HEAT. 


243 


moistening our finger, we can touch a hot flat-iron with 
impunity. The water assumes the above state, and 
thus protects the flesh from injury. Furnace-men 
often dip their moistened hands into molten iron. 
Probably the accounts handed down to us of per¬ 
sons walking unharmed over red-hot ploughshares 
are to be explained in this manner. 

Communication of Heat. 

Heat tends to diffuse itself equally among all 
surrounding bodies. There are three modes of dis=_ 
tribution. 

I. Conduction is the process of heating by the pas¬ 
sage of heat from molecule to molecule. Ex.: Hold one 
end of a poker in the fire, and the other end soon be¬ 
comes hot enough to burn the hand. Substances vary 
in their power of conduction. The denser bodies, as 
the metals, possess this property in the highest de¬ 
gree. Of the ordinary metals, copper is the best con¬ 
ductor. Wood is a poor conductor, especially “ across 
the grain.” Gases are the poorest conductors ; hence 
porous bodies, as wool, fur, snow, charcoal, etc., which 
contain within them large quantities of air, are excel¬ 
lent non-conductors. Refrigerators and ice-houses 
have double walls, filled between with charcoal, 
sawdust, or other non-conducting substances. Fire- 
safes contain plaster-of-paris. Air is so poor a con¬ 
ductor, that persons have gone into ovens, which 
were so hot as to cook meat and eggs which they 
carried in with them and laid on the metal shelves; 



244 


NATURAL PHILOSOPHY. 


yet, so long as they did not themselves touch any 
good conductor, they experienced little inconve¬ 
nience. Liquids are also poor conductors. Ex. : 
Hold the upper end of a test-tube of water in the 
flame of a lamp. The water nearest the blaze will 
boil, without the heat being felt by the hand. 

All adjacent objects have the same temperature, 
yet flannel sheets feel warm, and linen cold. A 
marble slab seems colder than the woollen carpet 
below it. If we touch an object colder than we are, 
it abstracts heat from us, and we say “ it feels cold 
if a warmer body, it imparts heat to us, and we say 
“ it feels warm.” These various effects depend en¬ 
tirely upon the relative conducting power of the 
different substances. Iron feels colder than feathers 
only because it robs us faster of our heat. 

II. Convection is tlie 
process of heating by circu¬ 
lation. —(1.) Convection of 
liquids. Place a little 
sawdust in a flask of wa¬ 
ter, and apply heat at the 
bottom. We shall soon 
find that an ascending 
and a descending cur¬ 
rent are established. 
The water near the lamp 
becoming heated, ex¬ 
pands and rises. The 
cold water above sinks to take its place. (2.) 


Fig. 174. 









HE A T. 


245 


Convection of gases. By testing with a lighted 
candle, we shall find that at the bottom of a door 
opening into cold air, there is a current setting in¬ 
ward, and at the top, one setting outward. The 
cold air in a room flows to the stove along the floor, 
is heated, and then rises to the ceiling. All methods 
of heating by hot-air furnaces depend upon the 
principle that warm air rises. 

III. Radiation is the process of heating by the 
transmission of rays in straight lines. All heat from 
the sun comes to the earth in this manner. A hot 
stove radiates heat. Bays of heat do not elevate 
the temperature of the media through which they 
pass. When the motion of the ether-waves is 
stopped, the effect is felt. Space is not warmed 
by the sunbeam. Meat can be cooked by radia¬ 
tion, while the air around is at the freezing-point. 
Radiation varies in different bodies, and in the same 
body under different circumstances. A rough un¬ 
polished surface is a better radiator than a smooth 
bright one. Extent of surface increases radiation. 
Air, vapor, and glass allow luminous rays of heat to 
pass through them readily. Thus the heat of the 
sunbeam easily penetrates our atmosphere, windows, 
etc. But the earth, and various objects on its sur¬ 
face, absorb and radiate the heat back again as 
obscure heat in long, slow waves. These have no 
power to pass the watery vapor in the air or through 
glass. They are thus entangled, and kept for our 
use. If the aqueous vapor were removed from our 


246 


NATURAL PHILOSOPHY. 


air, the earth would become uninhabitable, through 
the rapid radiation. On the desert of Sahara, where 
“ the soil is fire and the wind is flame,” the dry air 
allows the heat to escape through it so readily that 
ice is sometimes formed at night. The dryness of 
the air at great elevations accounts, in part, for the 
coldness which is there felt so keenly. 

Absorption and reflection are intimately connected 
with radiation. A good absorber is also a good 
radiator, but a good reflector can be neither. Snow 
is a good reflector but a poor absorber or radiator. 
Light colors absorb less and reflect more than dark 
colors. White is the best reflector, and blank the 
best absorber and radiator. 



The Steam-engine. 


When steam rises from water at a temperature of 
212° it has an elastic force of 15 lbs. per square 
inch. If the steam is confined and the temperature 
raised, the elastic force is rapidly increased. 

The steam-engine is a machine for using the elas¬ 
tic force of steam as a motive power. There are 
two classes of engines, the liigb-pressure and the low- 
pressure. In the former, the steam, after being em¬ 
ployed to do its work, is forced out into the air ; in 
the latter, it is condensed in a separate chamber by 
a spray of cold water. As the steam is condensed 
in the low-pressure engine, a vacuum is formed be¬ 
hind the piston; while the piston of the high-pres¬ 
sure engine acts against the pressure of the air. 


HEAT. 


247 


Fig. 175. 



The elastic force- of the steam must be 15 lbs. per 
square inch greater in the latter case. In the figure 
we have repre¬ 
sented the pis¬ 
ton and connect¬ 
ing pipes of an 
engine. The 9 
steam from the 
boiler passes 
through the pipe 
into the steam- 
chest, as indi¬ 
cated by the 
arrow. The slid¬ 
ing-valve worked by the rod li lets the steam into the 
cylinder alternately above and below the piston, 
which is thus made to play up and down by the ex¬ 
pansive force. 

The Governor is an apparatus for regulating the 
supply of steam. A B is the axis 
around which the heavy balls E 
and D revolve. When the ma¬ 
chine is going too fast the balls 
fly out by centrifugal force and 
shut off a portion of the steam ; 
when too slowly, they fall back, 
and, opening the valve, let on the 
steam again. 

The high-pressure engine in the form commonly 
used is shown in the frontispiece. A represents the 





























248 


NATUBAL PHILOSOPHY. 


cylinder, B the steam-chest, C the throttle-valve in 
the pipe through which steam is admitted from the 
boiler, D the governor, E the band-wheel by which 
the governor is driven, E the pump, G the crank, I 
the connector which is attached to a the cross-head, 
H the eccentric rod ( h in Eig. 175) which works the 
sliding-valve in the steam-chest, K the governor- 
valve, S the shaft by which the power is conveyed 
to the machinery. The cross-head, a, slides to 
and fro in a groove, and is fastened to the rod which 
works the piston in the cylinder A. The expansive 
force of the steam is thus communicated to a, thence 
to I, by which the crank is turned. The heavy fly¬ 
wheel, by its inertia, serves to render the movement 
of the machinery uniform. 

Meteorology. 

The air always contains moisture. The amount 
it can receive depends on the temperature ; warm 
air absorbing more, and cold air less. At 75° the 
vapor is sometimes so dense that in a cubic yard of 
atmosphere there is a cubic inch of water. At 50° 
half that quantity must be deposited. When the 
air at any temperature contains all the vapor it can 
hold, it is said to be saturated; any fall of tempera¬ 
ture will then cause a part of the vapor to he condensed . 
Most of the phenomena of rain, hail, dew, etc., de¬ 
pend on this principle. 

A change in density produces a change in temperature . 

Place a little tinder at the end of the piston of the 


HEAT. 


249 


fire-syringe shown in the figure. By forcing down 
the handle and compressing the air, sufficient 
heat is liberated to 
ignite the tinder. On 
the other hand, in 
experiments with the 
Air-pump we notice 
that as the air is rare¬ 
fied, a mist gathers in 
the glass receiver. This 
shows that the atmos¬ 
phere is cooled by its 
expansion, and so de¬ 
posits its vapor. The 
warm air from the 
earth ascending into 
the upper regions, is 
rarefied and cooled in 
the same manner. Its 
vapor is condensed into 
clouds, and often falls 
as rain. Owing to this 
expansion of the air, there is a gradual diminution 
of the temperature as the altitude is increased, at 
the rate of about 1° for every 300 feet. Even in 
tropical climates the tops of high mountains are 
covered with perpetual snow. At the equator the 
snow-line is 15,000 feet above the level of the sea. 
Should, however, a blast of cold air descend from a 
lofty height, it would become so condensed in fall- 
11 * 







/ 

4 5 o natural philosophy. 

ing, and its temperature thereby so elevated, that it 
would produce no injurious effect on vegetation. 

Dew. —The grass at night, becoming cooled by 
radiation, condenses upon its surface the vapor of 
the air.* Dew will gather most freely upon those 
objects that are the best radiators, as they will the 
soonest become cool. Thus grass, leaves, etc., wdiich 
need the most, get the most. It will not form on 
windy nights, because the air is constantly chang¬ 
ing and does not become cool enough to deposit its 
moisture. In tropical regions the nocturnal radia¬ 
tion is often so great as to admit of the formation of 
ice. In Bengal this is accomplished by exposing 
water in shallow earthen dishes resting on rice-straw. 
The most dew collects on a clear, cloudless night. 
In many countries, by its abundance, it supplies the 
place of rain, as in Chili, Arabia, etc. When the 
temperature of plants falls below 32°, the vapor is 
frozen upon them directly, and is called lioar-frost. 

Fogs are formed when the temperature of the air 
falls below the deio-point {%. e. f the temperature at 
which dew is deposited). They are found mainly 


* Dew was anciently thought to possess many wonderful prop¬ 
erties. Baths in this precious liquid were said to conduce greatly 
to beauty. It was collected for this purpose, and for the use of 
the alchemists in their weird experiments, by spreading fleeces of 
wool upon the ground. Laurens, a philosopher of the middle 
ages, claimed that dew is ethereal, so that if we should fill a lark’s 
egg with it and lay it out in the sun, immediately on the rising 
of that luminary, the egg will fly off into the air! This experi¬ 
ment is best performed with a goose’s egg. 



on low grounds and in the vicinity of rivers, ponds, 
etc., where the abundance of moisture keeps the air 
constantly saturated. 

Clouds differ from fogs only in their elevation in 
the atmosphere. They are formed when a “ warm, 
humid wind penetrates a cold air, or a cold wind a * 
warm, humid air.” Mountains are “ cloud-capped” 


Fig. 178. 



Different kinds of clouds—1 bird indicates the nimbus, 2 birds the stratus, 
3 birds the cumulus, and 4 birds the cirrus cloud. 


because the warm air rising from the valley is con¬ 
densed upon their cold summits. Clouds are con¬ 
stantly falling by their weight, but as they melt 
away in the warm air below, by condensation they 
increase above. 










252 


NATURAL PHILOSOPHY. 


The nimbus cloud is a dark-colored cloud from 
which rain is falling. 

The stratus cloud is composed of broad, widely- 
extended cloud-belts, sometimes spread over the 
whole sky. It is the lowest cloud, and often rests 
on the earth. It is the night-cloud. 

The cumulus cloud is made up of large cloud- 
masses looking like snow-capped mountains piled up 
along the horizon. It forms the summits of pillars 
of vapor, which, streaming up from the earth, are 
condensed in the upper air. It is the day-cloud. 
When of Small size and seen only near mid-day, it is 
a sign of fair weather. 

The cirrus (curl) cloud consists of light, fleecy 
clouds floating high in air. It is believed to be 
formed of spiculae of ice or flakes of snow. 

The cirro-cumulus is formed by small, distinct, 
rounded portions of the cirrus cloud, which separate 
from each other, leaving a clear sky between. 
Sailors call this a “ mackerel sky.” It accompanies 
warm, dry weather. 

The cirro-stratus is produced when the cirrus 
cloud spreads out into long, slender strata. It fore¬ 
bodes storms. 

The cumulo-stratus presents the peculiar forms 
called “ thunder-heads.” It is caused by a blend¬ 
ing of the cumulus with the stratus, and is a precur¬ 
sor of thunder-storms. 

' Rain is vapor condensed by the sudden cooling of 
the air in the upper regions. At a low temperature 


HEAT. 


253 


the vapor is frozen directly into snow . This may 
melt before it reaches the earth, and fall as rain. A 
sudden draught of cold air into a heated ball-room 
has sometimes produced a miniature snow-storm. 
The wonderful variety and beauty of snow-crystals 
are illustrated in the accompanying figure. 


Fig. 179. 



Winds are produced by variations in the tempera¬ 
ture of the air. The atmosphere at some point is 
expanded, rises, and colder air flows in to supply its 
place. This produces currents. The land and sea 
breezes of tropical islands are caused by the unequal 
specific heat of land and water. During the day the 
land becomes more highly heated than the water, 
and hence toward evening a sea-breeze sets in from 
the ocean. At night the land cools faster than the 


254 


NATURAL PHILOSOPHY. 


water, and so toward morning a land-breeze sets out 
from the land. Trade-winds are so named because 
by their regularity they favor commerce. A vessel 
on the Atlantic Ocean will sometimes, without shifting 
a sail, set steadily before this wind from Cape de 
Verde to the American coast. The air about the 
equator is highly heated, and, rising to the upper 
regions, flows off north and south. The cold air 
near the poles sets toward the equator to fill its 
place. If the earth were at rest this would make an 
upper warm current flowing from the equator, and 
a lower cold current flowing toward it. As the 
earth is revolving on its axis from west to east, the 
under current starting from the poles is constantly 
coming to a part moving faster than itself. It 
therefore lags behind. When it reaches the north 
equatorial regions it lags so much that it becomes a 
curreht from the northeast, and in the south equa¬ 
torial regions a current from the southeast. 

Oceanic Currents are produced in a similar man¬ 
ner. The water which is heated by the vertical sun 
of the tropics rises and flows toward the poles. The 
Gulf Stream, which issues from the Gulf of Mexico, 
carries the heat of the Caribbean Sea across the 
Northern Atlantic to the shores of Scotland and 
Norway. This tropical river flowing steadily through 
the cold water of the ocean, rescues England from 
the snows of Labrador. Should it by any chance 
break through the Isthmus of Panama, Great Britain 
would be condemned to eternal glaciers. 


HEAT. 


255 

Various Forms and Adaptations of Water. —The 
great specific heat of water adapts it to exercise a 
marked influence on climate. Warm winds sweep¬ 
ing northward meet the colder air of the temperate 
regions and deposit their moisture. The latent heat 
which the vapor absorbed in the sunny South is set 
free, to temper the severity of our snowy climate. 
Thus, aerial and oceanic currents constitute a water 
circulation which is a natural steam apparatus on 
the grandest scale, since it has a boiler at the equa¬ 
tor, and steam-pipes running over the entire globe. 
Water also equalizes the climate. It tends to pre¬ 
vent sudden changes of weather. In the summer it 
absorbs vast quantities of heat, which it gives off in 
the fall to moderate the approach of winter. In the 
spring the melting ice and snow drink in the warmth 
of the sunbeam, which else might prematurely coax 
forth the tender buds. Since so much heat is re¬ 
quired to melt the ice and snow, they dissolve very 
slowly, and thus prevent in a measure the disastrous 
floods which would inevitably follow, if they passed 
quickly into the liquid state. 

Water contains air, which is necessary for the 
support of fish. Just here occurs one of those 
happy coincidences which frequently startle the 
reverent searcher in Nature. Were water deprived 
of this air, it would be liable to explode at any 
moment when it happened to be heated much above 
212°. Every stove-boiler would need a thermom¬ 
eter. A teakettle would then require as careful 


NATURAL PHILOSOPHY. 


256 

watching as now to attend a steam-engine, and 
our kitchens would witness frequent and most dis¬ 
astrous explosions. As it is, when the tempera¬ 
ture rises above 212°, the extra heat passes off 
quietly and safely. Water expands with heat, like 
other liquids, and contracts, on cooling, down to 
39° F. Then it slowly expands until it reaches 32° 
F., when it freezes. The bursting of water-pipes 
and pails is a familiar example of this exception. 
Under the operation of the general law, the water 
at the surface radiating its heat and becoming 
chilled, would contract and fall to the bottom, while 
the warm water below would rise to the top. This 
process would continue until the freezing-point was 
reached, when the whole mass would instantly so¬ 
lidify into ice. Our lakes and rivers would thus 
freeze' solid every winter. This would be fatal to 
fish and aquatic vegetation. In the spring, the ice 
would not, as now, buoyant and light, float and melt 
in the direct sunbeam, but, lying at the bottom, 
would be protected by the non-conducting water 
above. The longest summer would not be sufficient 
to thaw the deeper bodies of water. Here we see 
another instance of prudent foresight. An exception 
is made to prevent these disastrous consequences.* 
The cold water expands and rises to the top, thus 
protecting the warm water beneath, while ice itself, 

* Certain metals—iron, bismuth, etc—are also an exception to 
the general law. This fact adapts them for castings. Is not this 
equally a thoughtful provision for our wants ? 



HEAT. 


257 


being a non-conductor, preserves the temperature 
of the water quite uniform during the entire winter. 

Water, in freezing, has a tendency to free itself 
from impurities. This furnishes a means of obtain¬ 
ing fresh water in Arctic regions. McClintock found 
that on each successive freezing the ice was purer, 
until, on the fourth time, he obtained drinking-water. 
If a barrel of vinegar freezes, we shall find the acid 
collected in a little mass at the centre of the ice. 

When the dew collects at night sufficiently to 
form a covering upon the plants, being a non-con¬ 
ductor, it stops further radiation of heat. Thus, by 
a nice provision, the effect of radiation checks the 
radiation itself, as soon as the wants of the thirsty 
vegetation are supplied. 

Water distills from the ocean and land as vapor, at 
one time cooling and refreshing the air, at another 
moderating its wintry rigor. It condenses into 
clouds, which shield the earth from the direct rays 
of the sun, and protect against excessive radiation. 
It falls as rain, cleansing the air and quickening 
vegetation with renewed life. It descends as snow, 
and, like a coverlet, wraps the grass and tender buds 
in its protecting embrace. It bubbles up in springs, 
invigorating us with cooling, healing draughts in the 
sickly heat of summer. It purifies our system, dis¬ 
solves our food, and keeps our joints supple. It 
flows to the ocean, fertilizing the soil, and floating 
the products of industry and toil to the markets of 
the world. (See Chemistry, pp. 56-63.) 


258 


NATURAL PHILOSOPHY. 


L“Practical Questions.—1. Why will one’s hand, on a frosty morning, 
freeze to a metallic door-knob sooner than to one of porcelain ? 2. Why does 
a piece of bread toasting curl up on the side exposed to the fire ? 3. Why do 
double windows protect from the cold ? 4. Why do furnace-men wear flan¬ 
nel shirts in summer to keep cool, and in winter to keep warm ? 5. Why do 
we blow our hands to make them warm, and our soup to make it cool ? 6. 
Why does snow protect the grass? 7. Why does water “boil away” more 
rapidly on some days than on others? 8. What causes the crackling sound 
in a stove when a fire is lighted ? 9. Why is the tone of a piano higher in a 
cold room than in a warm one ? 10. Ought an inkstand to have a large or a 
small mouth ? 11. Why is there a space left between the ends of the rails on 
a railroad track ? 12. Why is a person liable to take cold when his clothes 
are damp ? 13. What is the theory of corn-popping? 14. Could vacuum- 
pans be employed in cooking? 15. Why does the air feel so chilly in the 
spring, when snow and ice are melting ? 16. Why in freezing ice-cream do 
we put the ice in a wooden vessel and the cream in a tin one ? 17. Why 

does the temperature generally moderate when snow falls ? 18. What causes 
the singing of a teakettle. Ans. The escaping steam is thrown into vibra¬ 
tion by the peculiar shape of the spout. 19. Why does sprinkling a floor 
with water cool the air ? 20. How low a degree of temperature can be marked 
by a mercurial thermometer? 21. If the temperature be 70° F., what is it 
C. ? 22. Will dew form on an iron bridge? On a plank walk ? 23. Why 
will not corn pop when very dry? 24. The interior of the earth being a 
melted mass, why do we get the coldest water from a deep well ? 25. Ought 
the bottom of a teakettle to be polished? 26. Which boils the sooner, milk 
or water? 27. Is it economy to keep our stoves highly polished ? 28. If a 
thermometer be held in a running stream, will it indicate the same tempera¬ 
ture that it would in a pailful of the same water ? 29. Which makes the bet¬ 
ter “ holder,” woollen or cotton ? 30. Which will give out the more heat, a 
plain stove or one with ornamental designs ? 31. Does dew fall ? 32. What 
causes the “ sweating” of a pitcher? 33. Why is evaporation hastened in a 
vacuum? 34. Does stirring the ground around plants aid in the deposition 
of dew ? 35. Why does the snow at the foot of a tree melt sooner than that 
in the open field ? 36. Why is the opening in a chimney made to decrease 
in size from bottom to top ? 37. Will tea keep hot longer in a bright or a 
' dull teapot ? 38. What causes the snapping of wood when laid on the fire ? 
Ans. The expansion of the air in the cells of the wood. 39. Why is one’s 
breath visible on a cold day? 40. What gives the blue color to air ? Ans. 
The vapor it contains reflects the blue light of the sunbeam? 41. Why is 
light-colored clothing cooler in summer and warmer in winter than dark ? 
42. How does the heat at two feet from the fire compare with that at a dis¬ 
tance of four feet ? 43. Why does the frost remain later in-the morning upon 
some objects than upon others ? 44. Is it economy to use green wood ? 45. 
Why does not green wood snap ? 46. Why will a piece of metal dropped into 
a glass or porcelain dish of boiling water increase the ebullition ? 47. Which 
can be ignited the more quickly with a burning-glass, black or white paper ? 
48. Why does the air feel colder on a windy day ? 49. In what did the mir¬ 
acle of Gideon’s fleece consist? 50. Could a burning lens be made of ice ? 
51. Why is an iceberg frequently enveloped by a fog? 52. Would dew 
gather more freely on a rusty stove than on a bright kettle? 53. Why is a 
clear night colder than a cloudy one? 54. Why is nodew formed on cloudy 
nights ? 





** rhat power which, like a potent spirit, guide# 

The sea-wide wanderers over distant tides, 

Inspiring confidence where’er they roam, 

By indicating still the pathway home; — 

Through nature, quickened by the solar beam, 

Invests each atom with a force supreme, 

Directs the cavem’d crystal in its birth, 

And frames the mightiest mountains of the earth; 

Each leaf and flower by its strong law restrains, 

And binds the monarch Man within its mystic chains.” 

Huni 

















MAGNETIC ELECTRICITY. 


26 l 


Thales, one of the seven wise men, knew that 
when amber is rubbed with silk it will attract light 
bodies, as straw, leaves, etc. This property was 
considered so marvellous that amber was supposed 
to possess a soul. From the Greek name of this 
substance (elektron) our word electricity is derived. 
The electrical force manifests itself in five different 
forms—(1) Magnetic electricity; (2) Frictional or stati¬ 
cal electricity; (3) Galvanic , voltaic, or dynamic elec¬ 
tricity ; (4) Thermal electricity; (5) Animal electricity. 
These are intimately connected; their laws are 
strikingly related; they produce many effects in 
common ; and each can give rise to the other. 


MAGNETIC ELECTRICITY. 

Magnetism treats of the properties of magnets. 

A magnet is a body which has the power of at¬ 
tracting iron. The term is derived from the fact 
that an ore of iron possessing this property was first 
found at Magnesia, in Asia Minor. Natural magnets 
are generally known as lodestones (Saxon, laedan, 
to lead). The one worn by Sir Isaac Newton 
weighed only 3 grains, yet it was able to lift 746 
grains, or nearly 250 times its weight. Their power 
does not increase in proportion to their size. One 
brought from Moscow to London weighed 125 lbs., 
but could support only about 200 lbs. The artificial 
magnet consists of a magnetized steel bar; if straight, 


262 


NATURAL PHILOSOPHY. 


it is called a bar magnet; if bent into the shape 
of the letter U, a liorse-shoe magnet. A piece 
of soft iron, called the armature , is placed on the 
end. 

The Poles .—If we insert a magnet in iron-filings, 
they will cling chiefly to its extremities, which are 



Fig. 180. 



termed the poles. The magnetic force will be ex¬ 
erted even through an intervening body. Lay a 
sheet of paper on a magnet and sprinkle iron-filings 
upon it. They will collect in curious lines, the mag - 






MAGNETIC ELECTRICITY. 


263 




Fig. 181. 


netic curves , radiating from the poles. If a small bar 
magnet be suspended so as to swing freely, one pole 
will point toward the north and the other toward 


Fig. 182. 







264 


NATURAL PHILOSOPHY. 


the south. The north pole of the magnet is called 
the positive ( + ), and the south pole, the nega¬ 
tive (—). A magnet thus poised constitutes a mag - 
netic needle. If we hold a magnet near a magnetic 
needle, we shall find that the south pole of one at¬ 
tracts the north pole, and repels the south pole of 
the other. This proves the law of magnetic attrac¬ 
tion and repulsion—“ Like poles repel, and unlike 
poles attract .” 

Magnetic Induction is the power a magnet pos¬ 
sesses to develop magnetism in iron. If a piece of 
soft iron be brought near a magnet, it immediately 
assumes the magnetic state, which, however, it loses 
on being removed. In steel the change is perma¬ 
nent. The end of the bar next to the south pole of 
the magnet becomes the north pole of the new mag¬ 
net, and vice versa. When opposite states are thus 
developed in the opposite ends of a body, it is said 


to be polarized. 
Whenever any ob¬ 
ject is attracted 
by a magnet, it is 
supposed first to 
be made a mag¬ 
net (polarized) by 
induction, and 
then the attraction 


Fig. 183. 



consists merely in that of unlike poles for each 
other. Thus we may suspend from a magnet a 
chain of rings held together by magnetic attraction. 


MAGNETIC ELECTRICITY. 


265 

Each link is a magnet with its north and south 
poles. Each particle of the tuft of filings in Fig. 180 
is a distinct, perfect magnet. A magnet does not 
lose any force by inducing magnetism. It rather 
gains strength by the reflex influence of the new 
magnet. An armature acts in this manner to 
strengthen a magnet. If we break a magnet even 
into the smallest fragments, each part will have a 
north and a south pole. It is explained by sup¬ 
posing that every molecule of iron contains two 
kinds of electric force which neutralize each other. 
When magnetized they are separated, but do not 
leave the molecule in which they reside. Each 
molecule is thus polarized, the two halves assuming 
opposite magnetic states, as shown in the figure. 
The light half of each Fig 184 

little circle represents 
the positive, and the 
dark the negative side. All the molecules exert 
their negative force in one direction, and their posi¬ 
tive in the other. The forces thus neutralize each 
other at the centre, but manifest themselves at the 
ends of the magnet. 

How to make a Magnet.— The following method 
is an excellent one. Place the in- Fig . 185< 
ducing magnet on the unmagnet¬ 
ized one, as shown in the figure, 
and draw it from one end to the 
other several times, always carry¬ 
ing it back through the air in a 
circle to the starting-point. 


< B B BB » I* It 1*1* K 1* Id H Ii IIB I( B B 
o iu HI<BI<HB 18 T< i< i<x<i< k mm 









266 


NATURAL PHILOSOPHY. 


The Compass is a magnetic needle used by mari¬ 
ners, hunters, surveyors, etc. It is very delicately 
poised over a card on which the “ points of the 
compass” are marked. The needle does not often 
point directly N. and S. The “ line of no variation ,” 
as it is called, runs in an irregular course through 


Fig. 186. 



the United States from Cape Lookout across Lake 
Erie to Hudson’s Bay. East of this, the variation 
(declination) is toward the west, and west it is toward 
the east. The needle is subject also to daily and 
yearly variations, as well as those which require 
centuries to complete. The needle is, however, 
“ true to the pole,” although it shifts thus every 
hour in the day. It does so only in obedience to 
the laws which control its action. 








MAGNETIC ELECTRICITY. 2 gy 

Why the Needle points North and South. —The 
earth is a great magnet. This gives direction to 
the needle. Variations which are constantly taking 
place in the terrestrial magnetism produce corres¬ 
ponding changes in the needle. Suppose a magnet 
N S passing through the centre of a small globe. 
The needle s n will _ 

Fig. 187. Fig. 188. 

hang parallel to it, 
as in Fig. 187, its 
north pole being 
attracted by the s 
south pole of the 
magnet, and vice 
versa. If the globe 
be turned, Fig. 188, 
the north pole of 

the needle will bend— dip , as it is termed—down¬ 
ward. If the globe be turned in the other direction, 
the south pole of the needle will dip in the same 
manner. Similar phenomena are noticed in the 
compass. At the equator it is horizontal, but 
dips whenever taken north or south. An unmag¬ 
netized needle, if carefully poised, in our latitude, 
on being magnetized, immediately settles down, as 
if the north end were the heavier. This difficulty 
is remedied by making the north end of the needle 
lighter, and also by suspending a little weight upon 
the south end. The reverse is true in the southern 
hemisphere. 

A dipping-needle is poised as shown in Fig. 189. 



















268 


NATURAL PHILOSOPHY. 


Fig. 189. 


At the equator it hangs horizontally, but declines as 
it is carried north, until, at a 
place near Hudson’s Bay, as 
discovered by Captain Boss in 
1832, it becomes vertical. This 
point is called the North mag¬ 
netic pole. Strangely enough, 
it does not coincide with the 
geographical pole. The South 
magnetic pole has not yet been 
found. From the experiments 
we have made, we see that 
the end of the needle which 
points toward the N. pole of 
the earth, is really its S. pole. 

The Earth induces Magnetism.— All iron bars 
standing vertically (which in this latitude is not far 
from the line of the dip) possess slight magnetic 
properties. The upright parts of an iron fence, 
lightning rods, standards of chairs and desks, etc., 
on being tested by the magnetic needle, will be 
found to possess north polarity in the end next the 
ground, and south polarity in the other. The polar¬ 
ity of the lodestone has doubtless been caused in 
this manner in the lapse of ages. 





FRICTIONAL ELECTRICITY. 


269 


FRICTIONAL ELECTRICITY.* 


This is electricity developed by friction. One’s 
hair often crackles under a gutta-percha comb. A 
cat’s back, when rubbed in a dark room, emits 
sparks. In cold, frosty weather, a person, by shuf¬ 
fling about in his stocking-feet upon the carpet, can 
develop so much electricity in his body that he can 
ignite a jet of gas by simply applying his finger 
to it. 

The Electroscope is an instrument for detecting 
the presence of electricity. Bend a glass tube, and 
suspend from it a couple of Fig. 190 . 

pith-balls, as shown in the 
figure. Two strips of gold- 
leaf, hung in a glass jar 
(Fig. 191), form a more del¬ 
icate test. This instrument 
is so sensitive, that a slight 
flap of a silk handkerchief 
against the cover will cause the leaves to diverge. 

Two kinds of Electricity.— If a warm, dry glass 
tube (a lamp-chimney will answer) be rubbed with 
a silk handkerchief, a crackling sound is heard. If 
the tube be held near the face, we shall experience a . 
sensation like that given by a cobweb. The tube 



* The term static is applied to frictional electricity, and dy¬ 
namic to galvanic. The former indicates a force at rest; the 
latter, one in motion. 





270 


NATURAL PHILOSOPHY. 


will attract bits of paper, straw, feathers, etc. Pre¬ 
sent it to the pith-balls in the electroscope (Fig. 

Fig. i 9 i. 190). They will 

be attracted for 
an instant, and 
will then fly from 
the tube and 
from each other, 
apparently in 
the utmost dis¬ 
gust. Electrify 
a stick of seal¬ 
ing-wax and pre¬ 
sent it to the 
balls. They will 
act in the same 
manner. If we 
touch one ball 
to the excited 
glass, and the other to the excited wax, they will 
not, as before, fly from each other, but will rush to¬ 
gether at once. Present the glass to a ball: it will 
fly off when electrified. Present the glass again, and 
it will be repelled. Substitute the wax, and it will 
be attracted. Offer now the glass, and it will eagerly 
bound toward what it just before spurned. If the 
glass be held on one side of a ball and the wax on 
the other, it will fly between the two, carrying the 
electricity back and forth. From this we conclude (1), 
that there are two kinds of frictional as of magnetic 











FRICTIONAL ELECTRICITY. 


271 


electricity; and (2), like electricities repel each other , 
and unlike attract. Tlie electricity from tlie glass is 
termed vitreous or positive [+], and that from the 
wax, resinous or negative [ — ]. 

In the following list, each substance becomes 
positively electrified when rubbed with the body 
following it; but negatively, with the one preceding 
it. ( Ganot .) 


1. Cat’s fur. 

2. Flannel. 

3. Ivory. 

4. Glass. 


5. Cotton. 

6. Silk. 

7. The hand. 

8. Wood. 


9. Shellac. 

10. Resin. 

11. The metals. 

12. Sulphur. 


13. Caoutchouc. 

14. Gutta-percha. 

15. Gun-cotton. 


Theory of Electricity.— Of the nature of elec¬ 
tricity we know little. The positive and negative 
forces exist in every body in a state of equilibrium. 
When this is disturbed by friction, chemical action, 
etc., both are set free. We cannot develop one 
without the other. The opposite kinds manifest 
themselves at opposite parts of the surface, as in a 
magnet; it is therefore called a polar force. The 
slightest causes disturb the electric equilibrium. 
“ In cutting a slice of meat, there may pass between 
the steel knife and silver fork enough electricity to 
move the needle of a telegraph.” Yet the delioate 
balance of the opposing forces is so soon readjusted 
that we are unconscious of the change. 

Conductors and Insulators. —A body which'' al¬ 
lows the electric force to pass freely through it is 
termed a conductor; one which does not, is called a 
non-conductor , or insulator. Copper is one of the 


272 


NATURAL PHILOSOPHY. 


best conductors, and hence it is used in all electrical 
experiments. Glass is one of the best insulators. 
A body is said to be insulated when it is supported 
by some non-conducting substance, usually glass. 
The air, when dry, is a non-conductor, but when 
moist becomes a good conductor. Hence, the elec¬ 
tricity can be retained on an insulated body in a 
dry atmosphere, but is soon dissipated in a damp 
one. Electricity can be collected only by means of 
insulation. It can be developed by rubbing an iron 
rod, but is lost as fast as formed, by passing off 
through the metal to the hand. A glass rod does 
not conduct it to the body, so it is retained until it 
gradually dissipates in the air. The following list 
contains the most common conductors and insu¬ 
lators. 


Best Conductors. 
Metals. Vegetables. 

Charcoal. • Animals. 

Flame. Linen. 

Minerals. Cotton. 

Water. Dry Wood. 

Iron. Ice. 


Ice. 

Dry Wood. 
Caoutchouc. 
Dry Paper. 
Air. 

Silk. 


Glass. 

Wax. 

Sulphur. 

Amber. 

Shellac. 

Best Insulators. 


The electrical machine consists (1) of a glass 
wheel turned by a crank ; (2) of a pair of rubbers cov¬ 
ered with leather and spread with an amalgam (a 
mixture of tin, zinc, and mercury) which hastens the 
development of electricity; (3) of a comb or fork 
with fine points, since pointed bodies always favor 
the reception or dispersion of electricity ; (4) of a 
;prime conductor —a brass cylinder insulated by a 
glass standard so that the electricity cannot pass to 


FRICTIONAL ELECTRICITY. 


273 


the ground, and rounded at the ends so that it may 
not escape too rapidly into the atmosphere. 

At the commencement, the whole apparatus is in a 
state of equilibrium. By the friction, positive elec¬ 
tricity is developed on the glass, and negative on the 


Fig. 192. 



rubber. The negative escapes along the chain to the 
ground—the common reservoir. The positive, kept on 
the glass by the silk flaps, is carried around to the 
points. Here it attracts the negative electricity of the 
prime conductor, and the two forces, clashing together, 
form tiny sparks. The positive electricity naturally 
present in the prime conductor is thus left insulated, 
and the prime conductor is said to be charged with posi¬ 
tive electricity. If the negative conductor be insulated, 



































NATURAL PHILOSOPHY. 


the rubber will soon become charged with negative 
electricity, and the action of the machine will nearly 
cease. If the air be dry, the rubber freshly spread 
with amalgam, and the glass well rubbed with warm 
flannel, a sharp crackling noise will be heard, flashes 
will follow the wheel around, while sparks can be 
obtained from the prime conductor at a distance of 
several inches. The pith-ball electroscope, when 
charged and repelled by the prime conductor, will be 
quickly attracted by the rubber. This indicates the 
opposite electricities in them. 

Induction.— The influence of an electrified body 
over other bodies near it is termed electrical induc¬ 


tion. Thus, let a 
small insulated 
conductor be 
placed near the 
ball at the end of 
the prime con¬ 
ductor of an elec¬ 
trical machine. 
On charging the 
prime conductor 
the motion of the 
pith-balls will 


Fig. 193. 




show that the small conductor has also become 
charged. On testing with the electroscope, we shall 
find that the end next the positive prime conductor 
is negative, and the other end positive. As oppo¬ 
site electricities are thus developed at the opposite 







FRICTIONAL ELECTRICITY. 


275 


extremities of the conductor, it is polarized. Place 
several conductors, as shown in Fig. 194, connecting 

Fig. 194 . 



the copper ball at the right with the positive pole, 
and the one at the left with the negative pole of the 
electrical machine. The conductors will be charged 
and polarized by induction. 

Faraday's theory of induction assumes (1) that the 
electricity acts between the different molecules of a 
body, as between the different conductors in the last 
experiment—that each molecule becomes polarized, 
and in turn polarizes its neighbors, and that thus at 
last every molecule has opposite electricities on its 
opposite sides; (2) that the molecules of non-conduct¬ 
ors become polarized and retain their electricities, 
while the molecules of conductors become polarized 
and discharge their electricities into the adjacent 
molecules. The positive force thus passing from 
one molecule to another of a conductor accumulates 
at one end, and the negative, moving in the oppo¬ 
site direction, collects at the other end. Let P (Fig. 
195) represent the end of the positive conductor and 
N that of the small conductor in Fig. 193 ; and 












2j6 


NATURAL PHILOSOPHY. 


.rig. aw. 

! ©P 

dQd 

Odd 

Odd 

© 


Fig. 196. 


let the small circles represent molecules of air lying 
between the two—the lighter half indicating the 
Fig 195 positive and the darker half the negative side. 
The molecules of air being non-conducting, 
on being polarized from the influence of P, 
the prime conductor, retain their electrici¬ 
ties, but polarize each other in succession 
until N is reached. This being a conduct¬ 
ing body, its molecules impart their elec¬ 
tricity from one to the other, until the nega¬ 
tive electricity collects at one end and theu 

positive at the other. *._ 

Attraction and Repulsion.— Every case of attrap^ 
tion or repulsion is preceded by induction. “ The 
electric chime” illustrates 
this very prettily. It con¬ 
sists of three bells, two of 
which, c and b, are hung 
by brass chains, while the 
middle one is insulated 
above by a silk cord, and 
connected below with the 
earth by a chain. The 
balls between them are 
also insulated. The outer bells becoming charged 
with positive electricity from the prime conductor, 
polarize the balls by induction through the inter¬ 
vening air. The balls being then attracted to the 
bells, are charged and immediately repelled. Swing¬ 
ing away, they strike against the middle bell, dis- 










FRICTIONAL ELECTRICITY. 


277 


charge their electrical force, and are forthwith at¬ 
tracted again. Flying to and fro, they ring out a 
merry, electrical song. The dancing image is another 
illustration. It consists of a Fig. 197 . 

little pith-ball figure placed : 
between two metallic plates, 
the upper one hanging from 
the prime conductor, and the 
lower one connected with the 
earth. The dance is conduct¬ 
ed in a remarkably lively 
manner by alternate attrac¬ 
tion and repulsion.* 

The Leyden Jar consists of a glass jar coated 
inside and outside, nearly to the top, with tinfoil. 
It is fitted with a cover of Fig. m 

baked wood, through which 
passes a wire with a knob 
at the top, and below, a 
chain extending to the inner 
coating. The jar is charged 
by bringing the knob near 
the prime conductor of the 
electrical machine, while the 
outer coating communicates 
freely with the earth. Bright sparks will then leap 
in rapid succession to the inner coating, while simi- 

* A very slow motion should be given to the electrical wheel, 
and a pin thrust into the heel of the image will add much to the 
stamp of the tiny feet. 
























278 


NATURAL PHILOSOPHY. 


lar ones will pass off from the outer coating. The 
jar is discharged by holding one knob of the “ dis¬ 
charger” E, uj)on the outer coating, and the other 
upon the knob of the jar. The equilibrium will be re¬ 
stored with a sharp snap and a brilliant flash. Mi¬ 
nute particles are detached from the solid conduct¬ 
ors, and, burning, give color and brilliancy to the 
spark.* 

Explanation. —The charging of a jar with electrici¬ 
ty is entirely different from the process of filling 
one with water. The glass can as well be in the 
form of a pane. The only essentials are two con¬ 
ducting surfaces separated by a non-conducting body . 
The tinfoil acts only as a conductor to convey the 
electricity. This is finely illustrated by the “ Leyden 
jar with movable coatings,” which may be charged 
and then taken apart. Yery little electricity can be 
obtained from the glass, either of the tin coatings, 
or any two of the parts combined. When put to¬ 
gether again, the jar can be discharged in the usual 


* Professor Muschenbroek, of Leyden, discovered the princi¬ 
ple of the Leyden jar in the following curious way. While experi¬ 
menting, he held a bottle of water to the prime conductor of his 
electrical-machine. Noticing nothing peculiar, he attempted to 
investigate its condition. Holding the bottle with one hand, he 
happened to touch the water with the other, when he received a 
shock so unexpected, and so unlike anything he had ever felt 
before, that lie was tilled with astonishment. It was two days 
before he recovered from his fright. A few days afterward, in a 
letter to a friend, the Professor innocently remarked, that he 
would not take another shock for the whole kingdom of France. 



FRICTIONAL ELECTRICITY. 


279 

manner. Fig. 199 represents an enlarged section 
of the side of a Leyden jar: 1 indicates the inner 
coating; 2, the outer coating, and Fig. 190 . 

the circles between, the molecules 
of glass. The sparks of positive 
electricity from the prime con¬ 
ductor are distributed by the inner + 
coating, over the jar. The mole¬ 
cules of glass are polarized, while 
the outer coating becomes charged 
by induction with negative electricity. A quantity 
of positive electricity corresponding to the positive 
received by the inner coating escapes from the outer 
coating. If the jar be insulated so that this is unable 
to leave, the passage of the sparks will soon cease. If 
a finger be held near the outer coating, a spark will 
leap to it every time one enters the jar. The jar, 
therefore, when charged, contains no more electricity 
than in its natural state. It is only differently dis¬ 
tributed. 

The Electricity is on the Surface.— Each mole¬ 
cule within the surface of a solid, insulated conductor 
gives up its electricity with equal freedom in every 
direction; therefore it cannot become charged. 
Each molecule on the surface, however, receiving 
electricity from the particles behind it, and having 
non-conducting particles of air before it, must be¬ 
come charged. A bomb-shell can therefore hold as 
much electricity as a cannon-ball. 

The Effect of Points. —A pointed wire held near 


[ OCCi 
OC€* 

ooc* 

ocut 

OOC* 

OOC# 






280 


NATURAL PHILOSOPHY. 


the prime conductor will quietly draw off all its 
electricity, which will be seen'apparently clinging 
to the point like a little glowing star. If we fasten 
a pointed wire to the prime conductor, it will dis¬ 
charge the electricity in a brush of flame, silently, 
but so rapidly that even the pith-balls will,not reveal 
its presence in the conductor. If we hold one cheek 
near the point, we shall feel a current of air setting 
away from it. This is strong enough to deflect the 
flame of a candle. The particles of air near the point 
become polarized, are attracted, give up their negative 
electricity, and, being charged with positive electricity, 
are repelled; new ones take their place, and thus a 
current is established. The electric whirl , mounted on 
the prime conductor (Fig. 192), illustrates this action. 
As each molecule of air is repelled from a point, it 
reacts with equal force against the point. This is suf¬ 
ficient to set the light wire-wheel in rapid rotation. 

Atmospheric Electricity. —If, with the friction 
Vnnn a. small glass wheel, so much electricity is 



l, what immense quantities must be pro¬ 


duced by moving masses of air, clouds, etc.! Added 
to this, are the effects of heat, chemical changes, and 
the varied processes of nature—all of which disturb 
the electrical equilibrium. The air, therefore, is al¬ 
most constantly electrified. In clear weather it is 
in a positive state, but in foul weather it changes 
rapidly from positive to negative, and vice versa . 
Dr. Livingstone tells us that in South Africa the hot 
wind which blows over the desert is so highly elec- 


FRICTIONAL ELECTRICITY. 


28 I 


trifled, that a bunch of ostrich feathers held for a 
few seconds against it becomes as strongly charged 
as if attached to an electrical-machine, and will 
clasp the hand with a sharp, crackling sound. 

Lightning is only the discharge of a Leyden jar 
on the grand scale upon which Nature performs her 
operations.* Two clouds charged with opposite 
electricities, and separated by the non-conducting 
air, approach each other. When the tension be¬ 
comes sufficient to overcome the resistance, the two 
forces rush together with a blinding flash and ter¬ 
rific peal. The lightning moves along the line 
where there is the least resistance, and so describes 
a zigzag course. If we can trace the entire length, 
we call it chain-lightning; if we only see the flash 
through intervening clouds, it is sheet-lightning; and 
if it is the reflection of distant discharges, we term 
it heat-lightning . The report is caused by the clash¬ 
ing of the atoms of displaced air. The rolling of 
the thunder is produced by the reflection of the 

* The identity of lightning and frictional electricity was dis¬ 
covered by Franklin. He made a kite of a silk handkerchief, and 
fixed at the top a pointed wire. He elevated this during a thun¬ 
der-storm, tying at the end of the hemp string a key, and then 
insulating the whole by fastening it to a post with a long piece 
of silk lace. On presenting his knuckles to the key, he obtained 
a spark. So great was his joy, that he is said to have burst into 
tears. He afterward charged a Leyden jar, and performed other 
electrical experiments in this way. These attempts were attended 
with very great danger. A few years after, Prof. Richman drew 
in this manner from the clouds a ball of blue fire as large as a 
man’s fist. It struck him lifeless. 



2 82 


NATURAL PHILOSOPHY. 


sound from distant clouds. Sometimes the clouds 
and the earth become charged with opposite elec¬ 
tricities, separated by the non-conducting air. The 
spark from the discharge of this huge Leyden jar is 
a bolt that often causes fearful destruction. 


Fig. 200. 



The Aurora Borealis —“Northern lights”—is prob¬ 
ably caused by the passage of electricity through 
the rarefied atmosphere of the upper regions. It 
may be beautifully imitated, on a small scale, by 
passing a succession of sparks from the prime con¬ 
ductor through a long glass tube from which the air 
is nearly exhausted. The intimate relation between 
the aurora and magnetism is shown from the fact 
that the magnetic needle is disturbed when the au= 






FRICTIONAL ELECTRICITY. 


283 

rora is visible, and seems to tremble as the stream¬ 
ers dart to and fro. The telegraph is frequently 
worked by the current of electricity which passes 
along the wire on these occasions, thus for a time 
dispensing with the line-batteries. Geisslers Tubes 
are filled with rarefied gases, and then sealed. When 
a current of electricity is passed through them, the 
richest tints and variegated bands of color are exhib¬ 
ited. Gassiot's Cascade consists of a glass goblet 
coated with tinfoil on the inside. This is placed on the 
air-pump, and covered with a receiver which has a 
sliding-rod passing through the top. The air is then 
exhausted, and the rod brought into contact with 
the tinfoil. One conductor of the electrical-machine 
is connected with the rod, and the other with the 
pump-plate. The electricity will flow over the sides 
of the cup in a shower of soft undulations and deli¬ 
cate blue light. 

Lightning-rods were invented by Franklin. They 
are based on the principle that electricity always 
seeks the best conductor. The rod should be point¬ 
ed at the top with $ome metal which will not easily 
corrode. If constructed in separate parts, they should 
be securely jointed. The lower end should extend 
into water, or else deep into the damp ground, be¬ 
yond a possibility of any drought rendering the 
earth about it a non-conductor, and be packed about 
with ashes or charcoal. If the rod is of iron, it 
needs to be much larger than if of copper, which is 
a better conductor. Every elevated portion of the 


284 


NATURAL PHILOSOPHY. 


building should be protected by a separate rod 
Chimneys in which fire is constantly kept need espe¬ 
cial care, because of the ascending column of vapor 
and smoke. Water conductors, tin roofs, etc., should 
be connected with the damp ground or the light¬ 
ning-rod, that they may aid in conveying off the 
electricity. The value of a lightning-rod consists, 
most of all, in its power of quietly restoring the 
equilibrium between the earth and the clouds. By 
erecting lightning-rods, we thus lessen the liabilities 
of a sudden discharge. Providence has provided 
largely against this catastrophe. “ God has made 
a harmless conductor in every leaf, spire of grass, 
and twig. A common blade of grass, pointed by 
Nature’s exquisite workmanship, is three times more 
effectual than the finest cambric needle, and a single 
pointed twig than the metallic point of the best- 
constructed rod.” Every drop of rain, and every 
snow-flake, falls charged with the electric force, and 
thus quietly disarms the clouds of their terror. The 
balls of electric light, called by sailors “ St. Elmos 
fire” which sometimes cling to the masts and 
shrouds of vessels, and the flames seen to play 
about the points of bayonets, indicate the quiet 
escape of the electric force from the earth toward 
the clouds. 

Velocity of Electeicity. —The duration of the 
flash has been estimated at one-millionth of a sec¬ 
ond. Some idea of its instantaneousness can be 
formed from the fact that the spokes of a wheel re- 


FRICTIONAL ELECTRICITY . 


285 


volved so rapidly as to become invisible by daylight 
can be distinctly seen by the spark from* a Leyden 
jar. The trees swept by the tempest, when seen by 
a flash of lightning, seem motionless, while a can¬ 
non-ball, in swift flight, appears poised in mid-air. 
Wheatstone considered the velocity of lightning 
through a copper wire to be 288,000 miles per 
second. 

Effects of Frictional Electricity.—I. Physical. 
—Discharges from a large battery will melt rods of 
the various metals, perforate glass, split wood, mag¬ 
netize steel bars, etc. 1. Let a person stand upon 
an insulated stool and become charged from the 
prime conductor. His hair, through repulsion, will 
stand erect in a most ludicrous manner. On 
presenting his hand to a little ether contained in 



a spoon, a spark leaping from his extended finger 
will ignite it. If he hold in his hand an icicle, 
the spark will readily dart from it to the liquid. 
2. A card held between the knob of a Leyden jar 
and that of the discharger, will be punctured by the 
spark. 3. A piece of steel may be magnetized by 
the discharge from an ordinary Leyden jar. Wind 
a covered copper wire around a steel bar, as in Fig. 
201, or simply enclose a needle in a small glass 






286 


NA TUBAL PHILOSOPHY. 


tube around which the wire may be wound. On 
passing the spark through the wire, the needle will 
attract iron-filings. 4. When strips of tinfoil are 
pasted on glass, and figures of va¬ 
rious curious patterns cut from 
them, the electric spark leaping from 
one to the other presents a beauti¬ 
ful appearance. The diamond Ley¬ 
den jar and the spiral tube illustrate 
these effects in a brilliant manner. 
5. If a battery be discharged 
through a wire too small to conduct the spark, the 
electricity is changed to heat, and if sufficiently 
small, the wire will be fused into globules or dissi¬ 
pated in smoke. 

The fact that the electric force is thus converted 
into vibrations of heat and light, would seem to in¬ 
dicate that, like them, it is only a mode of motion. 

II. Chemical effects .—The “ electric gun” is filled 
with a mixture of oxygen and hydrogen gases. A 
spark causes them to combine with a loud explosion 
and form water. The sulphurous smell which ac¬ 
companies the working of an electrical-machine, 
and is noticed in places struck by lightning, is owing 
to the production of ozone, a peculiar form of the 
oxygen of the air. (See Chemistry, p. 38.) 

III. Physiological effects .—A very slight charge 
from a Leyden jar produces a contraction of the 
muscles and a spasmodic sensation in the wrist. A 
stronger one affects the body, and becomes painful 


Fig. 202. 










GALVANIC ELECTRICITY. 


287 


and even dangerous. The shock may be given to a 
large number of persons simultaneously by joining 
hands. The Abbe Nolle t once shocked in this way 
a regiment of 1,500 soldiers. 


GALVANIC ELECTRICITY. 

Galvanic or Voltaic electricity is produced by 
chemical action.* These names are given in honor 
of the two Italian philosophers who made the first 
discoveries in this branch of electricity. 

Galvani’s Discovery. —In the year 1790 Galvani 
was engaged in some experiments on animal elec¬ 
tricity. For this propose he used frogs’ legs as 
electroscopes. He had hung several of these upon 
copper hooks from the iron railing of the balcony, 
in order to see what effect the atmospheric elec¬ 
tricity might have upon them. He noticed, to his 
surprise, that when the wind blew them against the 
iron supports, the legs were convulsed as if in pain. 
After repeated experiments, Galvani concluded that 
this effect was produced by what he termed animal 
electricity, that this electricity was different from 
that caused by friction, and that he had discovered 
the agent by which the will controls the muscles. 

* The pupil, on recalling the definition of Natural Philosophy, 
will readily perceive that galvanic electricity is a connecting link 
between philosophy and chemistry. Its cause is chemical, while 
its effects are both chemical and philosophical. It is oftentimes 
ranked as a part of what is termed Chemical Physics. 



288 


NATURAL PHILOSOPHY . 



Volta’s Discovery.— Volta rejected the idea of 
animal electricity, and after 27 years of incessant 
study, discovered that the frog was not the source of 
the electricity, but “ only a moist conductor, and 
Fig. 203. was not as good as a wet rag for that pur¬ 
pose.” He applied this view to the con¬ 
struction of “ Volta’s pile.” This is com¬ 
posed of plates of zinc and copper, between 
which are laid pieces of flannel moistened 
with an acid or saline solution. We can 
easily form a simple voltaic pile by placing 
a silver coin between our teeth and upper lip, and a 
piece of zinc under our tongue. On pressing the 
edges of the two metals together, we shall perceive a 
peculiar taste, while a flash of light will pass before 
the closed eyes. Volta believed that the contact 
of two dissimilar metals develops electricity. His 
theory has given place to the chemical one which we 
shall now notice. 

The Simple Galvanic Circuit.— If we place a 
strip of zinc in a cup of water well acidulated with 
sulphuric acid (oil of vitriol), a 
chemical action will at once 
commence. Little bubbles of 
hydrogen gas will gather on the 
metal, while the zinc rapidly 
dissolves. If we now immerse 
the zinc in mercury, the surface 
will become as bright as a mir¬ 
ror. Replace the strip in the 


Fig. 204. 

/#h 










OALVANIG ELECTRICITY. 


289 

cup, and the acid will have no effect upon it. The 
reason of this action is not understood, but all zinc 
used in galvanic batteries is thoroughly and frequent¬ 
ly amalgamated in this manner. Now put a strip of 
copper in the acid. As long as the two metals remain 
separate no change takes place, but as soon as they 
touch, or are connected by wires as in the figure, 
chemical action begins, and the bubbles of hydro¬ 
gen gather upon the copper instead of the zinc as be¬ 
fore. The copper will not be changed, but the zinc 
will waste away. As soon as the wires are sepa¬ 
rated the action ceases, and, in the dark, a minute 
spark is seen. 

The ends of the wires are termed poles or elec¬ 
trodes. The copper pole is positive and the zinc 
negative. (These names may be easily remembered 
if we associate the p’s with copper and positive, and 
the n’s with zinc and negative.) Platinum strips are 
often fastened to the ends of the wires to act as 
electrodes , in order to withstand the corrosive liquids 
in which we may wish to place the poles. The join¬ 
ing of the wires is termed dosing the circuit , and 
separating them breaking the circuit. Two metallic 
plates combined in this manner form a voltaic pair. 
The two metals must be dissimilar (one positive and 
the other negative), and must be immersed in a 
liquid which is capable of producing a chemical ef¬ 
fect on only one of them. If both are equally acted 
upon, no current will be established, since the elec¬ 
tricity set freo by each will neutralize that developed 
. 13 


NATURAL PHILOSOPHY. 


29O 

by the other. The metal which sets free the elec¬ 
tricity is termed the positive, and the other the 
negative plate.* 

The chemical change which takes place in the 
voltaic pair may be very simply explained as follows: 
Each molecule of water is composed of an atom of 
oxygen and two of hydrogen ; the former unites with 
the zinc, forming oxide of zinc. The sulphuric acid 
combines with this, making sulphate of zinc, which 
dissolves in the water. The hydrogen being set free 
rises to the surface and escapes. (For the replace¬ 
ment theory, see Chemistry, p. 51.) 

Why the hydrogen comes offfrom the copper plate .— 
For simplicity of illustration, we shall suppose a row 
of water molecules t extending from 
the zinc to the copper plate. The 
negative oxygen of the. molecule of 
water nearest the positive zinc is at¬ 
tracted to that plate, while the posi¬ 
tive hydrogen is repelled. The atom 
thus driven off seeks refuge with the 
oxygen of the next molecule, and dis¬ 
possesses its hydrogen. This atom in turn robs the 
third molecule of its oxygen, and so on until the last 


* It should be noticed that the terms are reversed when applied 
to the plates and the poles. The zinc pole is negative, hut the 
zinc plate is positive; the copper pole is positive, but the copper 
plate is negative. We thus see that the plates when placed iu 
the liquid become polarized, as is represented in the figure. 

t In figure 205, a molecule of water is represented, for conven¬ 
ience, as consisting of only one atom of hydrogen and one of oxygen. 
















GALVANIC ELECTRICITY. 


291 


molecule is reached, when the atom of hydrogen, 
attracted by the negative copper, gives up to it its 
positive electricity, and then flies off into the air. 
Each atom of escaping hydrogen imparting its elec¬ 
trical force, adds to the current of electricity. 

The Voltaic Current .—The word “ current” is fre¬ 
quently used in electricity, but should not be under¬ 
stood to indicate the passage of a fluid, like the flow 
of water in a stream, but a mere transmission of the 
electrical force. Thus, if a long pipe were perfectly 
filled with water, a drop added at one end would 
thrust out a corresponding one at the other, which 
would not, however, be the identical one dropped 
in, since the force alone would traverse the length 
of the pipe. In the voltaic pair the current of posi¬ 
tive electricity sets out from the positive zinc 
through the liquid to the negative copper, thence 
through the wire around again to the zinc. If the 
circuit is broken, the current manifests itself at the 
copper pole. There is also a negative current pass¬ 
ing in the opposite direction; but, to avoid confusion, 
whenever the term current is used, the positive is 
intended. 

In galvanic as in frictional electricity, when the 
current passes through a conducting substance, as 
a wire, rod, etc., the force is transmitted, not on the 
surface, as is sometimes said, but through the entire 
thickness of the body. Each molecule, becoming po¬ 
larized and charged, discharges its force into the 
next molecule, and so on. The current thus moves 


292 


NATURAL PHILOSOPHY. 


by a rapid succession of polarizations and dis¬ 
charges of the molecules of the conductor. With 
what inconceivable rapidity must these successive 
changes take place in an iron wire to transmit the 
electric force, as in recent experiments, from San 
Francisco to Boston and return in one minute ! 

A battery consists of several voltaic pairs so com¬ 
bined as to increase the strength and steadiness of 
the electric current. 

Smee’s Battery. —Each cell consists of two plates 
of zinc and one of silver suspended between them. 

They are clamped together with 
screws and hung in a glass jar filled 
with dilute sulphuric acid. Since 
bubbles of hydrogen gas tend to 
collect on the smooth surface of the 
silver and interrupt the action, it is 
roughened with finely divided plati¬ 
num. 

Grove’s Battery is what is termed 
a “ two-fluid battery.” The outer 
glass jar contains dilute sulphuric acid, in which is 
placed a hollow zinc cylinder with a slit at the side 
to allow a free circulation of the liquid. 
The inner cup is of porous earthenware, 
and is filled with strong nitric acid (aqua 
fortis), in which is suspended a thin strip 
of platinum. 

Chemical change .—The water in the 
outer cup is decomposed, the oxygen 


Fig. 207. 



























GALVANIC ELECTRICITY. 


293 


uniting with the zinc and the sulphuric acid with 
both, to make sulphate of zinc. The hydrogen, how¬ 
ever, does not escape, as in Smee’s battery, but 
passes into the inner cup and tears apart the nitric 
acid, forming water and nitric oxide. The latter is 
at first absorbed by the liquid, but soon begins to 
escape in corrosive, blood-red fumes. If the zinc is 
properly amalgamated, no action will take place 
while the poles are separated, and the battery will 
remain quiescent, like a sleeping giant, but the in¬ 
stant the wires are connected the liquid will begin 
to boil with the evolution of the gas, while the elec¬ 
tric force will leap to the poles. (Rev. Chem., p. 47.) 

Advantages of this Battery. —(1.) The hydrogen 
does not collect on the negative (platinum) plate, 
since it is absorbed by the nitric acid. (2.) The 
liquid formed in the inner cup is an excellent con¬ 
ductor of ’electricity. (3.) Platinum is a more per¬ 
fect negative metal than copper, since it is not acted 
upon by the acid, and thus does not tend to start a 
counter-current; therefore platinum and zinc make 
a better voltaic pair than copper and zinc. (4.) The 
additional decomposition of the nitric acid sets free 
a great quantity of electricity. 

Bunsen’s Battery (Fig. 209) differs from Grove’s 
merely in substituting a carbon rod for the platinum 
strip in the inner cup. This, being an excellent 
conductor, answers the same purpose and is much 
cheaper. 

Daniell’s Constant Battery has an outer copper 


294 


NATURAL PHILOSOPHY. 


cup holding a solution of blue vitriol, and an inner 
porous cup containing a zinc rod and dilute sul¬ 
phuric acid. The sulphate of copper battery consists 
of a large zinc cylinder suspended in a copper jar 
containing a solution of sulphate of copper (blue 
vitriol). 

Quantity and Intensity.— A battery may develop 
a great quantity of electricity having a low degree 
of intensity, or a small quantity having a high in¬ 
tensity. Thus a cup of boiling water is intensely 
hot, while a hogshead full of that which is only 
blood-warm contains a great quantity of heat. The 
intensity of the electric force depends on the num¬ 
ber of cells ; the quantity, on their size. An intensity 
battery is formed by joining the zinc plate of one 
cell to the copper of the next; a quantity battery, 
by joining all the zinc and all the copper plates to¬ 
gether. The former method is preferable when 
great resistance is to be overcome. 

Comparison of Frictional with Galvanic Elec¬ 
tricity.— Frictional electricity is noisy, sudden, and 
convulsive ; galvanic is silent, constant, and power¬ 
ful. The one is a quick, violent blow ; the other a 
steady, uniform pressure. Intensity is the charac¬ 
teristic of the former, quantity of the latter. The 
lightning will leap through miles of intervening at¬ 
mosphere, while the galvanic current will follow a 
conductor around the globe, rather than jump across 
the gulf of a half inch of air. The most powerful 
frictional machine would be insufficient for tele- 


GALVANIC ELECTRICITY. 


295 


graphing; while despatches have been sent across 
the ocean with a tiny battery composed of “ a gun- 
cap and a strip of zinc, excited by a drop of water 
the bulk of a tear.” To decompose a grain of water 
would require over 6,000,000 discharges from a Ley¬ 
den jar—enough electricity to charge a thunder¬ 
cloud 35 acres in area; yet a few galvanic cups 
would tear apart that amount of water in perfect 
ease and silence. Faraday immersed a voltaic pair, 
composed of a wire of platinum and one of zinc, in 
a solution of 4 oz. of water and one drop of oil of 
•vitriol. In three seconds this produced as great a 
deviation of the galvanometer needle (Fig. 212) as 
was obtained by 30 turns of a powerful plate-glass 
machine. If this had been concentrated in one- 
millionth of a second, the duration of an electric 
spark, it would have been sufficient to kill a cat; yet 
it would require 800,000 such discharges to decom¬ 
pose a grain of water. 

I. Physical Effects of Voltaic Electricity.—1. 
Heat .—If a current of electricity be passed through 
a wire too small to conduct it readily, it is converted 
into heat. The poorer the conducting power of the 
wire, and hence the greater the resistance, the more 
readily the change is produced. With 10 or 12 of 
Grove’s cups several inches of fine steel wire may 
be thus fused, or even dissipated into smoke ; and 
with a powerful battery, several yards of platinum 
wire (the poorest conductor) may be made glowing 
hot. Torpedoes and blasts are fired on this prin- 


NATURAL PHILOSOPHY. 


296 

ciple. Two copper wires leading from the battery 
to the spot are separated in the powder by a short 
piece of small steel wire. When the circuit is com¬ 
pleted, the fine wire becomes red hot and explodes 
the charge. 

2. Light .—In closing or breaking the circuit we 
produce a spark, the size of which depends on the in¬ 
tensity of the current. With several cells, beauti- 
Fig. 208. fully variegated 

sparks are obtain¬ 
ed by fastening one 
pole to a file and' 
rubbing the other 
upon it. When 
charcoal or gas- 
carbon electrodes 
are used with a 
powerful battery, 
on slightly sepa¬ 
rating the points, 
the intervening 
space will be spanned by an arch of the most dazzling 
light. The flame, reaching out from the positive 
pole like a tongue, vibrates around the negative 
pole, licking now on this side and now on that. The 
heat is most intense. Platinum melts in it like wax 
in the flame of a candle, the metals burn with their 
characteristic colors, and even lime, quartz, etc., are 
fused. The effect is not produced by burning the 
charcoal points, since in a vacuum it is equally bril- 








GALVANIC ELECTRICITY. 


297 


liant. The cost of the electric light and the inten¬ 
sity of the illumination, which renders the shadows 
extremely dense, have prevented its general use. It 
is interesting to notice that in the battery there is 
zinc burning, i. e., combining with oxygen, but with¬ 
out light or heat; in the electric light the real force 
of the combustion is revealed. We may thus trans¬ 
fer the light and heat to a great distance from the fire. 

II. Chemical Effects.— 1. Decomposition of Wa¬ 
ter .—If the platinum electrodes are held a little dis¬ 
tance apart in a cup of water, little trains of tiny 

Fig. 209. 



bubbles will immediately begin to rise to the sur¬ 
face. If the copper poles are inserted, bubbles will 
pass off from the negative, but none from the posi¬ 
tive pole, since the oxygen combines with the copper 
wire. If the gases are collected, they will be found 
to be oxygen and hydrogen, in the proportion of two 
parts of the latter to one of the former. The theory 
13 * 









































NA TURAL PHIL 0SOPHY. 


298 

of the change is the same as that illustrated in Fig. 
205. It is a curious fact that the burning of an 
atom of zinc in the battery develops enough elec¬ 
tricity to set free an atom of oxygen at the positive 
pole. This indicates a very intimate relation be¬ 
tween chemical affinity and electricity—perhaps 
even their identity. 

2. Electrolysis (to loosen by electricity).—This is 
the process of the decomposition of compound bodies 
by the voltaic current. A substance which, like wa¬ 
ter, can be separated in this manner, is termed an 

electrolyte. 

Electro-positive and Electro-negative Substances .—In 
the electrolysis of compounds, their elements are 
found to be in different electrical conditions. Hy¬ 
drogen and most of the metals go to the negative 
pole, and (since unlike electricities attract) are elec¬ 
tro-positive. Oxygen, chlorine, sulphur, etc., go to 
the positive pole, and are therefore electro-negative. 
In the following list each substance is electro-nega¬ 
tive toward those which follow it, and electro-posi¬ 
tive toward those which precede. (Berzelius.) 


Electro-negative. 


1. Oxygen. 

12. Hydrogen. 

23. Iron. 

2. Sulphur. 

13. Gold. 

24. Zinc. 

3. Nitrogen. 

14. Platinum. 

25. Manganese. 

4. Chlorine. 

15. Mercury. 

26. Aluminum. 

5. Iodine. 

16. Silver. 

27. Magnesium. 

6 . Phosphorus. 

17. Copper. 

28. Calcium. 

7. Molybdenum. 

18. Bismuth. * 

29. Barium. 

8 . Tungsten. 

19. Tin. 

30. Lithium. 

9. Carbon. 

20. Lead. 

31. Sodium. 

10. Antimony. 

21. Cobalt. 

32. Potassium. 

11. Silicon. 

22. Nickel. 

Electro-pos 


GAL VANIC ELECTRICITY . 


2 99 

3. Electro-typing is the process of depositing metals 
from their solution by means of electricity. It is 
much used in copying medals, woodcuts, type, etc. 
An impression of the object is taken with gutta¬ 
percha or wax. The surface to be copied is brushed 
over with black-lead to render it a conductor. The 
mould is then suspended in a solution of sulphate 
of copper, from the negative pole of the battery; a 

Fier. 210. 



plate of copper is hung opposite on the positive pole. 
The electric current decomposes the sulphate of 
copper ; the metal goes to the negative pole and is 
deposited upon the mould, while the acid, passing to 
the positive pole, dissolves the copper, and thus pre¬ 
serves the strength of the solution. 

Duplicates of an engraved copper-plate are pre¬ 
pared in the following manner. The back of the 


















300 


NATURAL PHILOSOPHY. 


plate is rendered non-conducting by a coating of var¬ 
nish. The plate is then suspended in the solution. 
When the deposit of copper has reached the re¬ 
quired thickness, it is stripped off without difficulty. 
This, of course, represents the engraved plate in 
relief. If a facsimile is desired, a deposit is made 
in the same way upon the copy. Daguerreotype 
plates have been thus transferred without injury to 
the original. Leaves, insects, fruits, and even flow¬ 
ers, have been coated with copper by this wonderful 
process. 

While the plate is hanging in the solution there 
is no noise heard or bubbling seen. The most deli¬ 
cate sense fails to detect any movement. Yet the 
mysterious electric force is continually drawing 
particles of ruddy , solid copper out of the blue liquid , 
and, noiselessly as the fall of snowflakes, dropping 
them on the mould; producing a metal purer than 
any chemist can manufacture, spreading it with a uni¬ 
formity no artist can attain, and copying every line 
with a fidelity that knows no mistake. 

4. Electro-plating is the process of coating with silver 
or gold by electricity. The metal is deposited most 
readily on German silver, brass, copper, or nickel sil¬ 
ver. The last is a mixture of copper, zinc, and nickel, 
and is used for the best plated-ware. The vessels to be 
plated are thoroughly cleansed, and then hung in a 
solution of silver from the negative pole, while a plate 
of silver is suspended on the positive pole. In five 
minutes a mere “blush” of the metal will be depos- 


GALVANIC ELECTRICITY. 


301 


ited, which perfectly conceals the baser metal and 
is susceptible of a high polish. It is said that an 
ounce of silver can in this way be spread over two 
acres of surface. A well-plated spoon receives about 
as much silver as there is in a ten-cent piece. The 
only method of deciding accurately the amount de¬ 
posited is by weighing the article before and after 
being plated. A vessel is gold-lined by filling it 
with a solution of gold, suspending in it a slip of 
gold from the positive pole of the battery, and then 
attaching the negative pole to the vessel. The cur¬ 
rent passing through the liquid causes it to bubble 
like soda-rwater, and in a few moments deposits a 
thin film of gold over the entire surface. 

A simple Illustration in Plating .—Place in a large 
test-tube a silver coin with a little aqua-fortis. If 
the fumes of the decomposed acid do not soon 
rise, warm the liquid. When the silver is dissolved 
fill the tube nearly full of soft water. Next drop 
muriatic acid into the liquid until the white precipi¬ 
tate (chloride of silver) ceases to fall. When the 
chloride has settled, pour off the colored water 
which floats on top. Fill the tube again with soft 
water; shake it thoroughly; let it settle, and then 
pour off as before. Continue this process until the 
liquid loses all color. Finally, fill with water and 
heat moderately, adding cyanide of potassium in 
small bits as it dissolves, until the chloride is 
nearly taken up. The liquid is then ready for elec¬ 
tro-plating. Thoroughly cleanse a brass key, hang 


302 


NATURAL PHILOSOPHY. 


it from the negative pole of a small battery and 
suspend a silver coin from the positive pole. Place 
these in the silver solution, very near and facing 
each other. When well whitened by the deposit of 
silver, remove the key and polish it with chalk. In 
the arts the polishing is performed by rubbing with 
“ burnishers.” These are made of polished steel, 
and fit the surfaces of the various articles upon 
which they are to be used. 

III. Physiological Effects.— With a single cell 
no effect is experienced when the two poles are held 
in the hands. With a large battery a sudden twinge 
is felt, and the shock becomes painful and even dan¬ 
gerous, especially if the palms are moistened with 
salt-water, which increases the conduction. Rab¬ 
bits which had been suffocated for half an hour, 
have been restored to life by an application of the 
galvanic current. 


ELECTRO-MAGNETISM. 

Effect of a Voltaic Current on a Magnetic' 
Needle. —If a current of electricity be passed over 
a magnetic needle, the needle will turn and tend to 
place itself at right angles to the wire. If the wire 
be brought over and beneath the needle, it doubles 
the effect, and the play of the needle becomes a very 
delicate test of the presence and direction of the 
electric force. 


ELECTR O-MA QNETISM. 

3°3 


Fig. 211. 



The Galvanometer is an instrument for measur¬ 
ing the force and direction of an electric current. B 

Fig, 232. 














3°4 


NATURAL PHILOSOPHY. 


is a coil of wire, wound with thread to insulate it and 
compel the electricity to pass through the whole 
length ; the current is represented as entering at n 
and leaving at m. The silk cord, s, supports an 
astatic needle. This consists of two magnetic nee¬ 
dles, one over the graduated circle and the other 
within the coil, with the north pole of the one oppo¬ 
site the south pole of the other, so as to neutralize 
the attraction of the earth, and permit the combined 
needle to obey the will of the current alone. This 
affords a means of testing the faintest flow of 
electricity. 

Electro-magnets. —The voltaic current produces 
magnetism. If a current be passed through the 
wire shown in Eig. 201, the steel bar will be rendered 
magnetic. This shows the 
identity of the electricity 
from the voltaic battery with 
that from the Leyden jar. If 
the wire be wound around 
a bar of soft iron, as in Eig. 
213, the iron will instantly 
become a magnet which 
will grasp the armature 
with great force, but will as 
quickly lose its properties 
when the current is broken. Electro-magnets have 
been made that would lift 3,500 times their own 
weight. If the current be passed through a coil of 
insulated wire (a helix), as in Fig. 214, a rod 


Pig. 213. 





ELECTR O-ATA GNETISM. 


305 


of iron, when held below it, will 
be drawn up into it forcibly, as 
if pulled by a powerful spring; 
thus realizing in science the fabu¬ 
lous story of Mahomet’s coffin, which 
is said to have been suspended in 
mid-air. Here we see that not only 
does the soft iron within become 
magnetic, but also the coil itself. 
Bar-magnets are now r made by in¬ 
serting them in a large coil through 
which a powerful current is pass- 


Fig. 214. 





ing. 

Motion produced by Electricity .—If 
we reverse the direction of the cur¬ 
rent, it changes the poles of the 
magnet. Advantage is taken of this principle in 
order to produce continuous motion. Fig. 215 rep¬ 
resents Page’s rotating machine. 

It consists of an upright horse¬ 
shoe magnet, between the poles 
of which is a small electro-mag¬ 
net. Above this are two springs, 
which are so placed that, as the 
central rod revolves with the 
electro-magnet, the current passes 
through these springs, alternate¬ 
ly, to the wire coiled about the 
iron of the electro-magnet. The 
poles of the electro-magnet are 


Fig. 215. 











3 o6 


NATURAL PHILOSOPHY. 


thus changed twice with each revolution. The poles 
of the upright magnet attract the opposite poles of 
the electro-magnet, but as soon as they face each 
other the current is reversed, and they at once repel 
each other: the other poles are now attracted, but as 
they come together are repelled as before. A rapid 
motion is thus secured. The revolutions may rise 
as high as 2,500, making 5,000 reversals of the cur¬ 
rent in a minute. 

Electro-magnetic engines are constructed either on 
the principle that the magnet retains its power only 
while the current is passing, or that the poles are 
changed by reversing the current. They have been 
made of 8 or 10 horse-power, yet have never become 
of great practical value, because of the expense of 
the battery required to produce the electricity. The 
zinc which burns in the cell of the electric-engine 
is far more expensive than the coal which burns in 
the furnace of the steam-engine. 

The Electro-magnetic Telegraph depends on the 
same principle as the electro-magnet. A single 
wire is used to connect the two stations between 
which despatches are to be sent. The extremi¬ 
ties of the wire extend into the ground, and the 
earth completes the circuit. Each station has a 
hey and a receiver ; the former is used for sending 
messages, and the latter for receiving them. The 
key is shown in Fig. 216. The wire, P, leads from 
the battery ; L is the line-wire, and A connects with 
the receiver; a brass lever, h h> turns on an axis. 


ELECTR O-MA ONETISM. 


3°7 


A spring, r, elevates the lever, and keeps the pin, a , 
pressed down upon a little button just below, to 


which the wire, 
A, is attached. 
The key is now in 
a condition to re- 


Fis- 210. 


a 


n 



ceive a message, ggg 


The current from 
L passes through 

the lever k down the pin o, along the wire A, to the re¬ 
cording instrument, and thence to the earth, making 
the circuit complete. To send a message, the but¬ 
ton B is pressed down by the finger of the operator, 
so as to strike the button below it; the circuit is 
established there and broken beneath a. The cur¬ 
rent from the battery at the station now passes from 
P through h to L. The operator, by elevating or 
depressing B, can thus break or complete the circuit 
at his option. At the station where the despatch is 
received, the current passes, as we have seen, di¬ 
rectly into the receiver. This contains an electro¬ 
magnet, E. When the circuit is complete, the cur¬ 
rent, flashing through the coils of wire at E, at¬ 
tracts the armature, m. This elevates n y the other 
end of the lever, m n , and forces the sharp point, x , 
firmly against the soft paper, a. As soon as the 
circuit is broken, E ceases to be a magnet, and the 
spring, It, lifts the armature, drawing the point from 
the paper. Clock-work attached to the rollers at z 
moves the paper along uniformly beneath the point 




3°8 


NATURAL PHILOSOPHY. 


Fig. 217. 



x. When the circuit is completed and broken in- 
stantty, there is a sharp dot made on the paper. 
This is called e; two dots, i; three dots, 5; four 
dots, h. If the current is closed for a longer time, 
the mark becomes a dash ; this is t; two dashes, 
m ; a dot and a dash, a. 


Table of Morse’s Signs. 


a . — 

j_ 

8 ... 

b —_ 

k __ 

t — 

c . . . 

1 - 

U __ 

(1- 

m- 

V . -- 

e . 

n —. 

w -- 

f_ 

o . . 

X --- 

S - 

P . 

y •• .. 

h .... 

q- 

z . . . . 

i . . 

r . . . 

& . ... 


A skilful operator becomes so used to the sound 
that the clicking of the armature is perfectly intel¬ 
ligible. He uses, therefore, simply a “ sounder ,” i. e., 
a receiver without the paper and clock-work attach¬ 
ment, We thus see that the principle of the tele- 



















ELECTBO-MA GNETISM. 


3°9 


graph consists in closing and breaking the circuit at one 
station , and in making and unmaking an electro-magnet 
at the other . 

The Relay. —When the stations are more than fifty 
miles apart, the current becomes too weak to work 
the receiver. The relay uses the force of a local 



Fig. 218. 


battery for this purpose. L is the line-wire ; T the 
ground-wire ; c is connected with the positive pole ; 
Z with the receiver, and thence with the negative 
pole of the battery. The current passes in at L, 
traverses the fine wire of the electro-magnet, E, and 
thence passes out at T to the ground. The arma¬ 
ture A, playing to and fro as the current from the 
distant station darts through or is cut off, moves the 
lever p, which works on an axis at its centre, and is 
drawn back by the spring r. As A is attracted, p 
strikes against the screw n ; the current from C leaps 
up m to n, down p and through Z to the electro¬ 
magnet of the receiver and attracts its armature. 
The operator who sends the message simply com¬ 
pletes and breaks the circuit with the key , the ar- 








3 10 


NATURAL PHILOSOPHY. 


mature of the relay, at the station where the message 
is received, vibrates in unison with these movements, 
the receiver or sounder repeats them with greater 
force, and the second operator interprets their 
meaning. 

Magneto-electricity is that which is developed 
by means of magnetism. A common form of a ma¬ 
chine for this purpose is shown in Fig. 219. Coils 


Fig. 219. 



of wire are carefully insulated and wound around a 
small bar of soft iron, B, bent at right angles. This 
acts as the armature of a powerful horse-shoe mag¬ 
net, before the poles of which it is made to revolve. 
The soft iron becomes magnetic, and then induces 
electric currents in the coils. The poles are changed 
twice, and thus two opposite currents are induced 
in each revolution. By means of a break-piece 
the circuit is rapidly broken and closed. Severe 











ELECTRO-MA GNETISM. 


31 


shocks are thus produced, when the poles are grasped 
by the hands.* 

In Wilde's machine, the induced current from the 
coils is carried around a large electro-magnet, which 
is thereby excited to a high degree. The armature 
revolving before this furnishes the current which is 
used. A machine lately exhibited was driven by a 
steam-engine of 7-horse power. The poles were wire- 
rope, a quarter of an inch in diameter and 140 feet 
long. It produced an electric light dazzling as the 
noonday sun, throwing the flame of the street-lamps 
into shade at a quarter-mile distance. Its heat was 
sufficient to fuse a rod of iron a quarter of an inch 
in diameter and fourteen inches long, and could be 
felt fifty yards away. When one pole was inserted in 
the canal, and the other in a pool two hundred feet 
distant, the water was decomposed, oxygen gas bub¬ 
bling up at one electrode, and hydrogen at the 
other. 

Induced Currents. —Let two coils of wire be made 
to fit into each other, and carefully separated by 
insulators. If a current of electricity be passed 

* A Yankee once threw the industrial world of Europe into a 
wonderful excitement by announcing a new theoiy of perpetual 
motion based on the magneto-electric machine. He proposed to 
decompose water by the current of electricity, then bum the hy¬ 
drogen and oxygen thus obtained. In this way he would drive 
a small steam-engine, which, in turn, would keep the magneto¬ 
electric machine in motion. This would certainly be a splendid 
discoveiy. It would be a steam-engine which would prepare its 
own fuel, and, in addition, dispense light and heat to all around. 
(Helmholtz.) 



3 12 


NATURAL PHILOSOPHY. 


through the inner coil, it will induce a powerful 
secondary current, flowing in the opposite direction, 
in the outer coil. This soon ceases : on breaking 
the circuit, however, it will start again, but in the 
same direction as the primary current. The appa¬ 
ratus shown in Fig. 220 consists essentially of the 


Fig. 220. 



two coils just described. The primary current from 
a single cell is rapidly interrupted by means of a 
small electro-magnet. When this is magnetized, it 
attracts the armature, and thus the circuit is broken; 
the armature immediately springs back, and again 
completes the circuit. A bunch of iron wires may 
be inserted as a core in the inner coil. When the 
current passes, these become magnetized, and by 
induction largely strengthen the secondary current. 
This form is much used for medical purposes. 
Buhmlcorff's coil is constructed on the same prin¬ 
ciple. The largest coils often contain thirty to fifty 
miles of covered wire. Eitchie, of Boston, has de¬ 
vised many ingenious improvements which render 
the current extremely intense. His 15-incli coils 
will throw a quick succession of sparks, each fifteen 


THERMAL ELECTRICITY. 


313 


inches long, charge and discharge a Leyden jar, with 
a crack like that of a pistol, as rapidly as one can 
count, and perform the vacuum experiments in 
frictional electricity with a splendor and brilliancy 
no plate-machine can rival. 

THERMAL ELECTRICITY. 

As electricity can be changed into heat, in turn 
heat can be converted into electricity. A Thermo¬ 
electric pile consists of alternate bars of antimony 
and bismuth soldered together, as shown in Fig. 222. 
When mounted for Pig. 221 . Fig. 222 . 

use, the couples are 
insulated from each 
other and enclosed 
in a copper frame P. 

If both faces of the 
pile are equally 
heated, there is no 
current. The least variation of temperature, how¬ 
ever, between the two is indicated by the flow of 
electricity. Wires from a, the positive pole, and b, 
the negative, connect the pile with the galvanometer 
(Fig. 212). This constitutes one of the most delicate 
tests of the presence of heat. A tiny insect held 
against the face of the pile will move the needle. 
Strange, that minute quantities of heat become sen¬ 
sible only when they are converted into electricity, 
then into magnetism, and lastly into motion ! 





3*4 


NA TUB A L PHIL OSOPIIY. 


ANIMAL ELECTRICITY. 

Electric Fish have the property of giving, when 
touched, a shock like that from a Leyden jar. 
The torpedo and electrical eel are the most noted. 
The former is a native of the Mediterranean, and 
its shock was anciently much prized as a cure for 
various diseases. The latter is abundant in certain 
South American waters. A specimen of this fish, 
forty inches in length, was estimated by Faraday to 
emit a spark equal to the discharge of a battery of 
fifteen Leyden jars. The Indians are said to be ac¬ 
customed to drive herds of wild horses into the 
streams frequented by the fish. The horses are soon 
overpowered by the terrible shocks they receive, 
and so fall an easy prey to their pursuers. 


CONCLUSION. 


“ Science is a psalm and a prayer.”— Parker. 

Nowhere in nature do we find chance. Every 
event is governed by fixed laws. If we would ac¬ 
complish any result or perform any experiment, we 
must come into exact harmony with the universal 
system. If we deviate from the line of law by a 
hair’s breadth, we fail. These laws have been in 
operation since the creation, and all the discoveries 
of science prove them to extend to the most distant 



CONCLUSION. 


315 


star in space. A child of to-day amuses itself with 
casting a stone into the brook and watching the wi¬ 
dening curves: little antediluvian children could 
have done the same. A law of nature has no force 
of itself; it is but the manner in lohich force acts . 
We cannot create force. We can only take it as a 
gift from God. We find it everywhere in Nature. 
Matter is not dumb, but full of inherent energy. A 
tiny drop of dew sparkling on a spire of grass is in¬ 
stinct with power: Gravity draws it to the earth; 
Chemical Affinity binds together the atoms of hydro¬ 
gen and oxygen *, Cohesion holds the molecules of 
water, and gathers the drop into a globe; Heat keeps 
it in the liquid form ; Adhesion causes it to cling to 
the leaf. If the water be decomposed, Electricity 
would be set free ; and from this, Heat, Light, Mag¬ 
netism, and Motion could be produced. Thus the 
commonest object becomes full of fascination to the 
scientific mind, since in it reside the mysterious 
forces of Nature. 

These various forces can be classified either as 
attractive or repeUant. Under their influence the 
atoms or molecules resemble little magnets with 
positive and negative poles. They therefore ap¬ 
proach or recede from each other, and so tend to 
arrange themselves according to some definite plan. 
“ The atoms march in time, moving to the music of 
law.” A crystal is but a specimen of “ molecular 
architecture” built up by the forces with which mat¬ 
ter is endowed. 


3 16 


NATURAL PHILOSOPHY. 


No force can be destroyed. A hammer falls by 
the force of gravity and comes to rest, but its mo¬ 
tion as a mass is converted into a motion of atoms, 
and reveals itself to the sense of touch as heat. 
Thus force changes its form continually, but the eye 
of philosophy detects it and enables us to drive it 
from its various hiding-places still undiminished. It 
assumes Protean guises, but is doubtless essentially 
a unit everywhere. It may disappear from the 
earth ; still— 

“ Somewhere yet that atom’s force 
Moves the light poised universe.” 

This conversion of force is termed the “ Correlation 
of the Physical Forces.” It is the grandest law Na¬ 
ture offers for the contemplation of the human mind. 
What is the nature of force we cannot tell. We 
think it to be a mode of motion. Beyond this, all 
is mystery. 

The forces of Nature are strangely linked with our 
lives. Everywhere a Divine Hand is developing 
ideas tenderly and wondrously related to human 
needs. To the thoughtful mind all phenomena have 
a hidden meaning. 

“ To matter or to force 
The all is not confined ; 

Beside the law of things 
Is set the law of mind; 

One speaks in rock and star, 

And one within the brain, 

In unison at times, 

And then apart again. 

And both in one have brought us hither 
That we may know our whence and whithei 


CONCLUSION. 


3'7 


44 The sequences of law 
We learn through mind alone ; 

We see but outward forms, 

The soul the one thing known 
If she speak truth at all, 

The voices must be true 
That give these visible things, 

These laws, their honor due, 

But tell of One who brought us hither 
And holds the keys of whence and whither. 


“He in His science plans 
What no known laws foretell; 

The wandering fires and fixed 
Alike are miracle: 

The common death of all, 

The life renewed above, 

Are both within the scheme 
Of that all-circling love. 

The seeming chance that cast us hither 
Accomplishes His whence and whither. - 


\ 


NATIONAL SCHOOL APPARATUS, 


PHIL 0S01VI7CA L APPARATUS. 

UESIGNED TO ILLUSTRATE STEELE’S 14 WEEKS IN PHILOSOPHY. 

Set No. 1, — TPx-ice $125.00. 

The difficulty and inconvenience of procuring the articles necessary to 
practically illustrate these text-books, have induced the publishers of Peck's 
Ganot's , and Steele's Philosophies, to prepare sets of Apparatus complete 
enough to meet the wants of all. Set No. 2 is adequate to the performance 


of all the principal experiments in 
in almost any published. 


MECHANICS. 

Centrifugal Hoops . $7 00 

flocking Horse . 2 00 

Collision Halls . 12 00 

ELECTRICITY. 

5-in. Cylinder Machine.36 00 

Leyden Jar. Qt . 3 50 

Discharger . 4 OO 

Spiral Tube... . 6 OO 

Set Hells . 4 00 

Hlates for Images . 3 00 

PNEUMATICS. 

Air Hump and Heceiver.36 00 

Hemisqiheres . 14 OO 

Fountain in Tactic . 12 00 

Globe to Weigh Air . 3 50 

Hand t£ Hladder Glass.. 2 50 

HYDROSTATICS. 

Equilibrium . 5 00 

Hydrometer and Jar . 2 50 

Water Hammer . 1 60 

Hottle Imps . 1 50 

Syphon . O 50 


$156 60 


either of the text-books mentioned, or 


OPTICS. 

Compound Microscope . $20 00 

Concave <£■ Convex Mirrors. 4 OO 

Set lenses . 5 OO 

Magnifier . 3 OO 

Hrism . 2 50 

MAGNETISM ANO GALVANISM. 

Hot Hattery ... 6 00 

Electric Magnet . 4 00 

Magnet . 2 OO 

Har Magnet . 3 OO 

Magnetic Needle . 3 OO 

Hip Needle . 5 00 

CHEMISTRY, 

He tort Stand . 3 30 

Spirit Lamp . 2 25 

6 Hulb Tubes . 3 so 

Harometer Tube & Mercury 4 so 

6 Hupert’s Drops . 1 SO 

Funnel and Filters . 1 15 

Glass Tube Assorted . l 50 

Horous Cup . O 80 

Glass Tubes for Sound. . 1 60 

Compound Har . 3 OO 

Flask . O 60 


$81 40 


Set No. 3, — IPrice $500.00. 

Embracing all the articles in Set No. 1, with many additional fine instru¬ 
ments, and adequate to the performance of all the experiments in text-bookf 
generally. 

These Sets are securely packed in wooden boxes, and may be safely trans 
ported to any distance. Sent by express on receipt of price, or C. O. D. 

A. S. BARNES & CO., 

'll & 1'3 William Street, New York. 


P. O. Box 1672. 










































NOT.ES 


ON APPARATUS AND EXPERIMENTS. 


TAGE 

30. An Ivory ball from apparatus Fig. 33 can be used for 
this experiment. 

40. Half-dozen Rupert’s Drops. A glass funnel, pack of fil¬ 

ters, and 1 lb. animal charcoal. 

41. 1 lb. soft French glass tubing, assorted sizes. A 4 oz. 

alcohol lamp is also necessary. 

44 . Instead of the blue litmus, a solution of cabbage is good 
in this experiment. It is made by steeping purple 
cabbage-leaves in water, until the colored juice is ex¬ 
tracted. The funnel can be made of tin, by any tin¬ 
smith. 

46. A Grove’s cup, fitted with cork and tube. This may be 
supported with a wire tripod or any convenient device. 
The hydrogen preparation is described in Chemistry. 
51. Long tube for Guinea and Feather experiment. It may 
also be used to perform the experiment on page 282. 
54. Fig. 14 can be easily made. The rocking-horse, Fig. 1 c 
illustrates the principle more forcibly. 

58. Set of Pendulums, Fig. 19. 

59. This apparatus is not as essential as the last named, 

though very useful. 

60. The apparatus, Fig. 21, can be made by any carpenter. 

The pendulums are turned from hard wood, and hung 
on wire-hooks. 

71. Apparatus to explain 2d law of motion. 

77 Centrifugal-force apparatus, Fig. 32. 



NOTES ON APPARATUS AND EXPERIMENTS. 


^20 

79. Action and Reaction apparatus, Fig. 33. 

85. Model of the Mechanical Powers. 

104. Half-dozen tubes with bulbs, as in Fig. 67. 

106. Model of Hydrostatic Press. 

108. This series of tubes can be easily made by any teacher 

having the glass tubing and a spirit-lamp. 

109. Apparatus shown in Fig. 72. 

no. Apparatus shown in Fig. 73 or 74. The former is less 
liable to be injured by use. 

117. Hydrostatic balance and weights. This instrument is 

furnished with glass and brass disks to estimate the 
adhesion of solids and liquids. 

118. Hydrometer and jar. The jar may also be used in Figs. 8 

and 83. 

128. Model of Barker’s Mill. 

133. A table air-pump. This is the best and cheapest form 
of the air-pump. A barometer-gauge is a valuable 
addition. A condensing air-chamber, syringe, and 
jets, form a most valuable counterpart of the air- 
pump. By means of it, many instructive experiments 
in Hydraulics and Pneumatics may be performed. 

133. A copper flask and stop-cock, Fig. 93. 

134. Two black Cartesian imps. 

136. Hand-glass. Magdeburg Hemispheres. 

137. Upward-pressure apparatus. 

138. Apparatus shown in Fig. 101. 

138. Barometer-tube, open at both ends. 3 lbs. of mercury. 
142. Model of forcing and lifting pumps. 

146. A glass siphon, with tube for exhausting the air. 

155. Sound in vacuo. A much cheaper apparatus than the 
one shown in the figure will answer the purpose of this 
experiment. The bell may be suspended by a cord 
and rung by a sliding rod, or by simply tilting the 
pump. The effect will not be as complete as that 
stated in the text 

172. Figs. 123 ana 125 can be made by any ingenious pupil, 
and will afford profitable amusement. Trv them. 


.vOTES ON APPARATUS AND EXPERIMENTS. 


321 


17j. A vibrating-plate and violin-bow, Fig. 125. 

183. Glass tubes for singing flames. The experiment is mosi 
satisfactory when an apparatus like that shown in the 
figure is employed. The tube may, however, be held 
by the hand. The beaks of broken retorts make ex¬ 
cellent tubes. 

194 A concave and convex mirror in one frame. 

200. A set of small lenses. 

201. A large double convex lens, mounted. 

£05 Mounted prism. The lens just mentioned can be used 
for the recomposition of the light, but a more striking 
way is to use a painted disk, to be attached to the ap¬ 
paratus for centrifugal force, Fig. 32. 

215. A compound microscope, with mounted objects. 

219. Magic lantern. This is capable of almost unlimited use, 
if means can be procured to purchase mounted slides 
illustrative of principles in Astronomy, Geology, Bot¬ 
any, Physiology, etc. 

234. A compound bar to illustrate unequal expansion of metals. 

240. A Florence Flask, Fig. 171. This flask may be used 
also for Fig. 174. 

240. Water-hammer or Pulse-glass. 

261. Bar and horse-shoe magnets. 1 lb. iron-filings. 

263. Small horizontal and dipping needles. 

273. Electrical machine, electric whirl, and brass chain. An 

insulating stool may be extemporized with an ordinary 
stool, by setting the legs in glass tumblers. 

274. Small insulated conductor. 

276. Electric chime. 

277. Dancing-image plates, and pith-ball image. 

278. Leyden jar with movable coatings. 

277. Leyden jar and discharger. 

286. Spiral tube and diamond Leyden jar. 

(All the experiments in Galvanism and Electro-magnet¬ 
ism can be performed with a large sulphate of copper 
battery, except the decomposition of water and the 
electric light. This battery is cheap, and very con- 


^22 NOTES ON APPARATUS AND EXPERIMENT^. 


venient to use. For the other experiments a battery 
of 5 to 12 of Grove’s Cups will answer, though the 
electric arch cannot be well exhibited with less than 
40 to 60 cups. 

304. An electro-magnet. 

305.. A lifting coil. 

305. Page’s rotating machine. 

307-9. Model of a telegraphic machine. 

310. Magneto-electric machine in a box. 

312. Electro-magnetic machine, Fig. 220. 

313. Thermo-electric pile and galvanometer, Figs. 221 & 223 , 


EF* Priced lists of the above apparatus will be furnishea 
on application to A. S. BARNES CO., 

hi and 113 William St., New York, 



QUESTIONS 


The following questions are those which the author has 
used in his own classes, both as a daily review and in examina¬ 
tion. A standing question, which has followed every other 
question, has been: “Can you illustrate this?" Without, 
therefore, a particular request, the pupil has been accustomed 
to give as many practical examples as he could, whenever he 
has made any statement or given any definition. The figures 
refer to the page of the book. 

INTRODUCTION. —Define matter. A body. A substance. 
Name and define the two kinds of properties which belong to 
each substance. 

14. The two kinds of change. What is the principal dis¬ 
tinction between Philosophy and Chemistry ? Mention some 
phenomena which belong to each. Why are these branches 
intimately related ? 

15. Name the general properties of matter. Define magni¬ 
tude. Size. Why is it necessary to have a standard of meas¬ 
ure ? Whence were the ancient standards derived ? Give the 
history of the English standard. 

16. Is the American yard an exact copy of the English ? 
Have we any national standard ? Give an account of the 
French system. By what name is this system commonly 
known? Is either of these systems founded on any natural 
standard ? Why is it desirable to have such a standard ? 

17. Define Impenetrability. Give some apparent excep¬ 
tions, and explain them. Define Divisibility. 

19. Is there any limit to the divisibility of matter ? Explain 
the Atomic Theory. What use has it ? 

20. How do animalculae illustrate this subject? Under a 
powerful microscope how would chalk-marks appear ? 

21. Define Porosity. Is the word porous here used in its 
common acceptation ? Define a molecule. An atom. Com¬ 
pare the size of an atom with that of a pore. 



3 2 4 


QUESTIONS IN PHILOSOPHY. 


25. Define Inertia. Does a ball, when thrown, stop itself? 
Why is it difficult to start a heavy wagon ? Why is it danger¬ 
ous to jump from the cars when in motion ? 

26. Define Indestructibility. Did the earth, at its creation, 
contain the same quantity of matter it does now? 

27. Name the specific properties of matter. Define Duc¬ 
tility. How is iron wire made? 

28. Platinum wire ? Gilt wire ? What is said of brass wire ? 
Define Malleability. Describe the manufacture of gold-leaf. 

29. Is copper malleable ? Define Tenacity. Name and 
define the three kinds of Elasticity. Illustrate the elasticity 
of compression as seen in solids. 

30. In liquids. In gases. What is said about the relative 
compressibility of liquids and gases ? Compare air with water. 

31. Illustrate the elasticity of expansion as seen in solids, 
liquids, and gases. Define Elasticity of Torsion. What is a 
Torsion balance? Define Hardness. Does this property de¬ 
pend on density ? 

32. Define Density. Define Brittleness. Is a hard body 
necessarily brittle ? Why are feathers light and lead heavy ? 

Molecular Forces. —Define a molecular force. What 
two opposing forces act between the molecules of matter? 
How is this shown ? What is the repellant force ? Name the 
attractive forces. Which of these belong to Philosophy ? 

Cohesion. —Define. What are the three states of matter? 
Define. How can a body be changed from one state to an¬ 
other ? Show that cohesion acts only at insensible distances. 
Explain the process of welding. 

37. Why cannot all metals be welded ? Why do drops of 
dew, etc., take a globular form ? Why do not all bodies have 
this form ? 

38. Illustrate the tendency of matter to a crystalline struc¬ 
ture. Has each substance its own form? 

39. Why is not cast-iron crystalline ? Why do the axles of 
cars become brittle after use ? Describe the process of tem¬ 
pering and annealing. 

40. Explain the Rupert Drop. 

Adhesion. —Define. What is the theory of filtering through 
charcoal ? 

41. Of what use is soap in making bubbles? Define Capil¬ 
lary Attraction. Why will water rise in a glass tube, while 
mercury will be depressed? Is a tube necessary to show 
capillary attraction ? What is the law of the rise in tubes ? 


QUESTIONS IN PHILOSOPHY. 


3 2 5 

42-3. Give practical illustrations of capillary action. Why 
will not old cloth shrink as well as new, when washed ? 

44. What is the cause of solution? Why is the process 
hastened by pulverizing ? Tell what you can about gases dis¬ 
solving in water. Why does the gas escape from soda-water 
as soon as drawn ? Why do pressure and cold favor the solu¬ 
tion of a gas ? Describe the diffusion of liquids. 

45 -7. Of gases. The osmose of liquids. Of gases. Why 
do rose-balloons lose their buoyancy ? 

Gravitation. —How does Gravitation differ from Cohesion 
and Adhesion ? What is the law of gravitation ? Why does 
a stone fall to the ground? Will a plumb-line near a moun¬ 
tain hang perpendicularly? Why do the bubbles in a cup of 
tea gather on the side ? 

49. How is the earth kept in its place? Define Gravita¬ 
tion. Gravity. Weight. Give the three laws of weight. 

50-2. What is a vertical or plumb-line? Give the four 
laws of falling bodies. Describe the “ guinea and feather 
experiment.” What does it prove ? 

53. Give the equations of falling bodies. How can the time 
of a falling body be used for determining the depth of a well ? 
How does gravity act upon a body thrown upward ? What 
velocity must be given to a ball to elevate it to any point ? 
How high will it rise in a given time ? When it falls, with 
what force will it strike the ground ? 

54-6. Define the Centre of Gravity. The line of direction. 
The three states of equilibrium. How may the centre of 
gravity be found ? Give the general principles of the centre of 
gravity. Describe the leaning tower of Pisa. 

57. Give some physiological applications of the centre of 
gravity. Why do fat people always walk so erect ? 

58-9. Define the Pendulum. Arc. Amplitude. What are 
isochronous vibrations. Give the four laws of the pendulum. 
Who discovered the first law ? How ? 

60-2. What is the centre of oscillation ? How is it found ? 
Describe the pendulum of a clock. How is a clock regulated ? 
Does it gain or lose time in winter ? Describe the gridiron 
pendulum. 

63. Name the various uses of the pendulum. 

MOTION. —Define motion, absolute and relative. Rest. 
Velocity. Force. What are the resistances to motion ? Tell 
what you can about friction. Why does oil diminish friction ? 

68 . What uses has friction ? What law governs the resisfc 


questions in philosophy- 


326 

ance of air or water? What is the striking force? (p. 81, 
Prob. 35.) What is the tendency of gravity? Define Mo¬ 
mentum. 

69. Show that motion is not imparted instantaneously. 

70-1. Give the three laws of motion and the proof of each. 
If a ball be fired into the air when a horizontal wind is blow¬ 
ing, will it rise as high as if the air were still ? Define com¬ 
pound motion. 

72. Define the “parallelogram of forces.” The resultant. 
How can the resultant of two or more forces be found ? Give 
practical illustrations of compound motion. 

73. What is the “resolution of forces?” Show how one 
vessel can sail south and another north, driven by the same 
westerly wind. 

74-5. Explain how a kite is raised. Explain the towing of 
a canal-boat. Define circular motion. 

76. Apply the principle of circular motion to the revolu¬ 
tion of the earth about the sun. 

77. Show when the centrifugal force becomes strong enough 
to overcome the force of Cohesion, Adhesion, Gravity. What 
effect does the revolution of the earth on its axis have upon 
all bodies on the surface? What would be the effect if the 
rotation were to cease ? Describe the action of the centrifugal 
force on a hoop rapidly revolved on its axis. 

78. Give practical illustrations of action and reaction. If a 
bird could live, could it fly in a vacuum ? 

79. Define reflected motion. Give its law. 

80. How is curved motion produced ? Is perpetual motion 
practicable ? 

The Mechanical Powers. —Name and define the ele¬ 
ments of machinery. Do the “powers,” so called, produce 
force ? What is the law of Mechanics ? Illustrate the law. 

86-7. Describe the three classes of levers. The law of 
equilibrium. 

88. What is the advantage peculiar to each class ? Describe 
the steelyard as a lever. What effect does it have to reverse 
the steelyard ? 

89. Describe the arm as a lever. Would a lever of the 
first class answer the purpose of the arm ? What is a bent 
lever ? 

90. Describe the compound lever. The wheel and axle. 

91. The capstan. Give the law of equilibrium. What is 
the advantage of the wheel and axle ? 


QUESTIONS IN PHILOSOPHY . 


3 2 7 


92. Describe a system of wheel-work. At which arm of 
the lever is the P. applied ? 

93-4. Describe the various uses of the inclined plane. Its 
law of equilibrium. What velocity does a body acquire in 
rolling down an inclined plane ? Give illustrations. 

95. Describe the screw. Its uses. Its law of equilibrium. 
How may its power be increased ? What limit is there ? 

96. Describe the wedge. Its uses. Its law of equilibrium. 
How does it differ from that of the other powers ? 

97. Describe the pulley. The use of fixed pulleys. Is 
there any gain of P. in a fixed pulley? 

98. The use of a movable pulley. Describe a movable pul¬ 
ley as a lever. 

99. Give the general law of equilibrium in a combination 
pulleys. What part of the force is lost by friction? 



Hydrostatics. —Define. What liquid is taken as the 
type ? What is the first law of liquids ? Explain. Illustrate 
the transmission of pressure by water. 

105. Show how water is used as a mechanical power. 

106. Describe the hydrostatic press. Give its law of equi¬ 
librium. 

107. What are the uses of this press? What pressure is 
sustained by the lower part of a vessel of water, when acted on 
by gravity alone ? How does this pressure act ? 

108. Give the four laws which depend on this principle, and 
illustrate them. What is the weight of a cubic foot of sea¬ 
water ? Fresh water ? What is the pressure at two feet ? 

109. Give illustrations of the pressure at great depths. 

no. Describe the hydrostatic bellows. Its law of equi¬ 
librium. 

hi. What is the hydrostatic paradox? Give illustrations. 
Give the principle of fountains. How high will the water rise? 

112-3. How do modern engineers carry water across a 
river? Did the ancients understand this principle? Give the 
theory of the Artesian well. 

114. Give the rule for finding the pressure on the bottom 
of a vessel. On the side. 

115. Define the water-level. Is the surface of water hori¬ 
zontal ? If it were, what part of an approaching ship would 
we see first ? Describe the spirit-level. Define specific gravi¬ 
ty. What is the standard for solids and liquids? For gases r 

116^7. Explain the buoyant force of liquids. What is 


QUESTIONS IN PHILOSOPHY. 


3 ** 


Archimedes's law? Describe the “cylinder and bucket ex¬ 
periment." What does it prove? 

118. Give the method of finding the specific gravity of a 
solid. A liquid. Suppose the solid is lighter than water 
and will not sink, what can you do? A ns. Tie a heavy 
solid to it, and then make allowance for this in calculating the 
specific gravity. Explain the hydrometer. 

119. How can you find the weight of a given bulk of any 
substance ? The bulk of any given weight ? The exact vol¬ 
ume of a body ? 

120-1. Illustrate the action of dense liquids on floating 
bodies. Why will an iron ship float on water? Where is 
the centre of gravity in a floating body? How do fish sink at 
pleasure? 

' XJhTDRAULlCS.—Define. To what is the velocity of a jet 
equal ? How is the velocity found? Give the rule for finding 
the quantity of water which can be discharged from a jet in a 
given time. 

124. What is the effect of tubes? Tell something of the 
flow of water in rivers. 

125-7. Name and describe the different kinds of water¬ 
wheels. Which is the most valuable form ? What is the 
principle of the Turbine ? Describe Barker’s Mill. 

128-9. How are waves produced ? Explain the real motion 
of the wave. How does the motion of the whole wave differ 
from that of each particle ? How is the character of waves 
modified near the shore ? 

130. What is the extreme height of “mountain waves?" 

1 Define like phases. Unlike phases. A wave-length. What 
i is the effect if two waves with like phases coincide? With 
\ jmlike phases? What is this termed? 

^ Nv ' , ~Pn£UMATICS.—D efine. What principles are common to 
liquids and gases? What gas is taken as the type? De¬ 
scribe the air-pump. Can a perfect vacuum be obtained in 
thi, way? Prove that the air has weight. 

134-5. Show its elasticity and compressibility. Describe 
the bottle-imps. What principles do they illustrate? Show 
the expansibility of the air. 

136. Describe the experiments with the hand-glass. The 
Magdeburg hemispheres. What do they prove ? 

137. Show the upward pressure of the air. The buoyant 
force of the air. Would a pound of feathers and a pound 0/ 


QUESTIONS IN PHILOSOPHY. 


3 2 9 

lead balance, if placed in a vacuum ? On what principle does 
a balloon rise ? 

138-140. What is the amount of the pressure of the air? 
Describe the experiment illustrating this. Where do these 
figures apply? Describe how the pressure of the air con¬ 
stantly varies. 

141. Give Mariotte’s law. Describe the barometer. Its 
uses. Are the terms “fair,” “foul,” etc., often placed on the 
scale, to be relied upon ? 

142. Why is mercury used for filling the barometer? De¬ 
scribe Otto Guericke’s barometer. 

143-4. Describe the action of the lifting-pump. The force- 
pump. The fire-engine. 

145-6. The siphon. Explain its theory. 

147. The pneumatic inkstand. What was the view of the 
ancients concerning the pressure of the air? Tell the story of 
Galileo. What opposing forces act on the air? 

148. How high does the air extend ? How does its density 
vary? 

Acoustics. —Define. Name and define the two senses of 
this word. May not “light,” “heat,” etc., be used in the 
same way ? Illustrate the formation of sound by vibrations. 

152. Show how the sound of a tuning-fork is conveyed 
through the air. Report of a gun. The sound of a bell. 
The human voice. Define a sound-wave. A wave-length. 
In which direction do the molecules of air vibrate? In what 
form do the waves spread ? Can a sound be made in a vacuum ? 

155. Can a sound come to the earth from the stars ? How 
do sounds change as we pass above or below the sea-level ? 
Upon what does the velocity of sound depend ? Why is this? 

156. At what rate does sound travel in the air? In water? 
In iron ? What effect does temperature have on the velocity 
of sound ? Describe Biot’s experiment in the water-pipes of 
Paris. Do all sounds travel at the same rate ? 

157-8. How does the velocity of sound enable us to deter¬ 
mine distance? Upon what does the intensity of sound de¬ 
pend? At what rate does it diminish? Why? State where¬ 
in the laws of sound are similar to those of other phenomena. 
What does this uniformity prove? Explain the speaking-tube. 

159. The ear-trumpet. The speaking-trumpet. What is 
the refraction of sound ? 

160. Define reflection of sound. What is the law? Give 


33° 


QUESTIONS IN PHILOSOPHY. 


curious instances of reflection. What is the shape of a 
whispering-gallery ? 

161. Illustrate the decrease of sound by repeated reflection. 
Why are sounds more distinct at night than by day ? What 
is a resonance ? 

162. Is it desirable to have a door or a window behind a 
speaker? What causes the “ ringing” of a sea-shell? When 
is an echo heard? When is the echo repeated? 

163. What is the difference between noise and music? 
Upon what does pitch depend? 

164-6. Describe the siren. How is it used to determine the 
number of vibrations in any sound ? How is the octave of 
any note produced ? How can we ascertain the length of the 
wave in sound? 

167. What length of wave produces the low tones in music ? 
The high tones? Give the illustration of the locomotive 
whistle. When are two tones in unison ? How can we find 
the length of the wave in any musical sound? 

168. What is the length of the wave in a man’s voice in 
common conversation ? How can two sounds produce silence ? 
What is this effect termed? 

169. Illustrate interference by means of a tuning-fork. De- 
£o*ibe the vibration of a cord. 

t^o-i. Describe the sonometer. What is the object of the 
wooden box ? Give the three laws of the vibration of cords. 
What is a node ? 

172. Describe the experiments illustrating the formation of 
nodes. 

173. What are acoustic figures? Nodal lines? 

175. What is the fundamental tone of a cord? A har¬ 
monic ? What causes the difference in the sound of various 
instruments? Does a bell vibrate in nodes? The violin- 
case ? A piano sounding-board ? 

e 176. Give the fractions representing the relative rates of 
vibration of the different notes of the scale. How is the sound 
produced in wind-instruments ? 

177. How is the sound-wave started in an organ-pipe? In 
a flute ? What determines the pitch ? 

178. What are sympathetic vibrations ? Describe the ear. 

179. What is the object of the Eustachian tube? Is there 
any opening between the external and internal ear? What 
effect does it have on the hearing to increase or diminish the 


QUESTIONS IN rniLOSOPIIY. 


33 1 

pressure of the aii ? How does a concussion sometimes cause 
temporary deafness ? How can this be remedied ? 

180. What are the limits of hearing? Does the range vary 
in different persons ? What sounds are generally most acutely 
heard ? 

181. Are there probably sounds in nature we never hear? 
Has nature a tendency to music? What causes the “whisper¬ 
ing of the pines ?” 

182. What is the key of nature? What are sensitive flames? 
How can a flame be made to sing? What causes the song? 

Optics. —Define. A luminous body. A non-luminous 
body. A medium. A transparent body. A translucent 
body. An opaque body. A ray of light. Show that neither 
air nor water is perfectly transparent. Why is the sun’s light 
fainter at sunset than at mid-day ? 

188. Define the visual angle. Show how distance and size 
are intimately related. Give the laws of light. Do they re¬ 
semble those of sound? 

189. What is the velocity of light? How is this proved? 
Explain the undulatory theory of light. 

190. How does light-motion differ from sound-motion ? 
What is diffused light ? Why are some objects brilliant and 
others dull ? Why can we see a rough surface at any angle, 
and an image in the mirror at only a particular one ? Would 
a perfectly smooth mirror be visible? How does reflection 
vary? Define mirrors. Name and define the three kinds. 
What is the action of each on rays of light ? What is the 
general principle of mirrors ? 

192-3. Why is an image in a plane mir*or symmetrical? 
Why is it reversed right and left? Why is it as far behind 
the mirror as the object is before it? If you sit where you 
cannot see another person’s image, why cannot that person s rt e 
yours ? Why can we often see in a mirror several images of 
an object ? Why can we see these best if we look into the mir¬ 
ror very obliquely? Why is an image seen in water inverted? 

194. When the moon is near the meridian why can we see 
the image in the water at only one spot ? When do we see a 
tremulous line of light? Define the focus. Centre of curva¬ 
ture. 

195. Describe the image seen in a concave mirror. Why 
is it inverted when we stand between the centre of curvature 
and the principal focus ? Why is it larger than life when we 
stand within the principal focus, and smaller than life when 
we stand without the centre of curvature ? 


S3 2 


QUESTIONS IN PHILOSOPHY. 


196. What are conjugate foci? Describe the image seen in 
a convex mirror. Why is it smaller than life ? Why can it 
not be inverted like one seen in a concave mirror ? A?is. Be¬ 
cause the rays do not cross each other. 

197. Define total reflection. Define Refraction. Does the 
partial reflection of light as it passes from one medium to 
another of different density have a parallel in sound ? Why is 
powdered ice opaque while a block of ice is transparent? 
Give illustrations of refraction. 

198. Why does an object in water appear to be above it* 
true place ? What is the general principle of refraction ? 

199. Give the laws of refraction. Describe the path of a 
ray through a window-glass. Is the direction of objects 
changed? Describe the path through a prism. 

200. Name and describe the different kinds of lenses. What 
is the effect of a double convex lens on rays of light ? 

201. What is this kind of lens often called? Describe the 
image. Why is it inverted after we pass the principal focus ? 
Why is it decreased in size ? 

202. What is the effect of a double concave lens on rays ol 
light ? Describe the image. Why can it not be inverted like 
one through a double convex lens ? Describe the images seen 
in the large vases in the windows of drug-stores. What is a 
mirage ? 

203. Give its cause. 

204. How is the solar spectrum formed? Name the seven 
primary colors. Show that these seven will form white light. 
What other opinions are held ? 

205. Why are the rays separated? What is meant by the 
dispersive power of a prism ? What substance possesses this 
property in the highest degree ? 

206. What three classes of rays compose the spectrum? 
Do artificial lights differ in their proportion of these rays? 
What color, also, predominates? A ns. Yellow. Why does 
the window of a photographer’s dark room sometimes contain 
yellow glass ? Define complementary colors. How can they 
be seen? What is the effect of complementary colors when 
brought in contrast? (In Fig. 153, opposite colors are com¬ 
plementary.) Ought a red flower to be placed in a bouquet 
by an orange one ? A pink or blue with a violet one ? Why 
do colors seen by artificial light appear differently than by 
daylight—as yellow seems white, blue turns to green, etc. ? 

207-8. Describe Newton’s rings. How are these explained 


QUESTIONS IN PUILOSOTUY. 


333 

according to the wave-theory ? What can you say about the 
length of the waves ? 

209. State the analogy between color and pitch in music. 
Why is grass green ? When is a body white ? Black ? What 
causes the play of color in mother-of-pearl? In soap-bubbles? 
In the scum on stagnant water ? In thin layers of mica or 
quartz ? What is a tint ? 

210. Define diffraction. What is double refraction ? What 
are the two rays termed ? What is polarized light ? 

211. How does a dot appear through Iceland spar? What 
other methods of polarizing light? Give some illustrations 
and practical uses of polarized light. 

212-3. How is the rainbow formed ? Why must it rain and 
the sun shine at the same time, to produce the bow? Why is 
the bow in the sky opposite the sun ? How many refractions 
and reflections form the primary bow ? The secondary ? How 
many colors can one receive from a single drop ? Why is the 
bow circular ? 

214. How are halos formed? What is the cause of the 
“sun’s drawing water?” Explain spherical aberration. 

215. Chromatic aberration. Its remedy. What is the 
meaning of the word microscope ? Describe the simple mi¬ 
croscope. The compound microscope. How is the power of 
a microscope indicated ? Do we see the object directly in a 
microscope ? Why is the object-lens made so small and so 
convex ? 

216-8. What is the meaning of the word telescope? De¬ 
scribe the reflecting telescope. The refracting telescope. 
What is the use of the object-lens ? The eye-piece ? Is the 
image inverted ? Describe the opera-glass. 

219. The stereoscope. The magic lantern. How are dis¬ 
solving views produced ? 

220. Describe the Camera. The structure of the eye. 

222. The formation of an image on the retina. The ad¬ 
justment of the eye. The cause of near and far sightedness. 
The remedy. Why do old people hold a book at arm’s-length ? 

223. Illustrate the duration of an impression. Why are we 
not sensible of darkness when we wink ? Why can we not see 
the fence-posts when we are riding rapidly? Describe color¬ 
blindness. What is the range of the eye ? 

Heat. —Define luminous heat. Obscure heat. A dia- 
thermanous body. Cold. Gases and vapors. Show the in¬ 
timate relation between light and heat 


334 


QUESTIONS IN PHILOSOPHY. 


228-9. What is light? How do the three classes of rays 
in the solar spectrum differ? What effect does each of these 
produce ? What is the theory of heat ? Why can we not see 
with our fingers or taste with our ears ? At what rate does 
nerve-motion travel? How long does it take a tall man to 
find out what is going on in his foot? What is meant by the 
quality of heat ? Does this find any analogy in sound ? 

230. Name the sources of heat. Describe and illustrate 
each of these. Can force be destroyed? If apparently lost, 
what becomes of it ? What is Joule’s law? 

232. Define latent, sensibLe, and specific heat. 

233. Explain the paradox, “ that freezing is a warming pio- 
cess and thawing a cooling one.” Explain the action of a 
freezing mixture. Why does heat expand and cold contract ? 
What do you say as to the uniformity of the expansion of 
solids, liquids, and gases ? 

234. Illustrate the expansion of solids. Is it better to buy 
alcohol in summer or in winter ? 

235. What is the thermometer? Describe it. Describe 
the process of filling and grading. 

236. The F., C., and R. scales. Tell what you can about 
liquefaction. Of a solid. Of a gas. In one case sensible 
heat becomes latent, in the other latent heat becomes sensi¬ 
ble—why is this ? 

237. Give the theory of vaporization. Distillation. Since 
rain comes from the ocean, why is it not salt ? 

238-9. Describe the theory of boiling. What is the boil¬ 
ing point ? Do all liquids boil at the same temperature ? 
What would be the effect, if this were the case ? Upon what 
does the boiling-point depend ? Why does salt-water boil at 
a higher temperature than fresh-water ? Why will milk boil 
over so easily ? Why will soup keep hot longer than boiling 
water ? Does the air, dissolved in water, have any influence 
on the boiling-point? (Page 247.) Can you measure the 
height of a mountain by means of a tea-kettle and a ther¬ 
mometer ? Show how cold water may be used to make warm 
water boil. 

240. At what temperature will water boil in a vacuum ? 
Why ? Describe the water-hammer and the pulse-glass. 

241. Can we heat water in the open air above the boiling- 
point ? What becomes of the extra heat ? What is the latent 
heat of water ? Upon what principle are buildings heated by 
steam ? Have you ever seen any steam ? Define evaporation. 
Does snow evaporate in the winter ? What can be done to 


QUESTIONS IN PHIL OS0PHY. 


335 

hasten evaporation? Why is a saucepan made broad? Why 
do we cool ourselves by fanning? Why does an application 
of spirits to the forehead allay fever ? Why does wind hasten 
the drying of clothes? Describe a vacuum-pan. Why is 
evaporation hastened in a vacuum ? 

242. Why is evaporation a cooling process ? How is ice 
manufactured in the tropics ? What is the spheroidal state ? 

243. Name and define the three modes of communicating 
heat. Give illustrations showing the relative conducting 
power of solids, liquids, and gases. What substances are the 
best conductors? 

244. Is water a good conductor ? Air ? What is the prin¬ 
ciple of ice-houses ? Fire-proof safes ? Why do not flannel 
and marble appear to be of the same temperature ? Is ice 
always of the same temperature, or is some ice colder than 
others? Describe the convective currents in heating water. 
Where must the heat be applied ? Where should ice be ap¬ 
plied in order to cool water ? 

245. Describe the convective currents in heating air. Upon 
what principle are hot-air furnaces constructed ? Ought the 
ventilator at the top of a room to be opened in winter ? 
At the bottom ? Is space warmed by the sunbeam ? Show 
how the glass in a hothouse acts as a trap to catch the 
sunbeam. Does the heat of the sun come in through our 
windows ? Does the heat of our stoves pass out in the same 
way? A ns. It does, but only through absorption by the 
glass, and not by direct radiation from the fire. Show how 
the vapor in the air helps to keep the earth warm. The 
top of a mountain is nearer the sun, why is it not warmer? 
(Page 249.) Why does ice form at night on the Desert of 
Sahara ? Explain the relation between absorption and reflec¬ 
tion. Is a dusty boot hotter to the foot than a polished one ? 
What is the elastic force of steam at the ordinary pressure of 
the air ? What is the difference between a high-pressure and a 
low-pressure engine? Which is used for a locomotive? Why? 

247. Describe the governor. What is the object of a fly¬ 
wheel ? 

* 248. How does the capacity of the air for moisture vary ? 
What is the principle on which dew, rain, etc., depend? 
Show that a change in density produces a change in tempera¬ 
ture. What effect does this have on the temperature of eleva¬ 
ted regions ? 

250. How is dew formed ? Upon what objects will it collect 


QUESTIONS IN PHILOSOPHY. 


33 6 

most readily ? Why will it not form on windy nights? Is a 
heavy dew a sign of rain? Ans. Yes, because it shows that 
the moisture of the air is easily condensed. The “sweating 
of a pitcher ?” What is frost? A ns. Frozen dew. Whj 
will a slight covering protect plants from frost? Ans. Be* 
cause it prevents radiation. Why is there no frost on cloudy 
nights ? Ans. The clouds act like a blanket, to prevent radia¬ 
tion and keep the earth warm. What is a fog ? 

251. How does a fog differ from a cloud? Why are moun¬ 
tains “ cloud-capped?” Why do clouds remain suspended in 
the air, contrary to gravity ? 

252. Describe the different kinds of clouds. Describe the 
formation of rain. 

253-4. Snow. Winds. Land and sea breezes. Trade- 
winds. Oceanic currents. Tell about the Gulf Stream. Ex¬ 
plain the influence which water has on climate. Of what 
practical use is the air in water ? 

256. Describe the exception which exists in the freezing of 
water. Why is this made ? Describe the two processes by 
which pure water can be obtained. How is an excessive de¬ 
posit of dew prevented ? 

Electricity. —Give the origin of this word. Name the 
different kinds of Electricity. Define Magnetism. A Magnet. 
A natural magnet. An artificial one. A bar-magnet. 

262. A horse-shoe magnet. The poles. The magnetic curves. 

264. Describe a magnetic needle. What is the law of mag¬ 
netic attraction and repulsion? Define magnetic induction. 
Explain it. When is a body polarized ? Give some illustra¬ 
tions of induced magnetism. 

265. Does a magnet lose any force by induction? How do 
you explain the fact that if you break a magnet each part will 
have its N. and S. pole ? Describe the process of making a 
magnet. On what principle will you explain this ? 

266. Describe the compass. Is the needle true to the 
pole ? What causes it to vary ? What is the line of no varia¬ 
tion ? Declination ? 

267. Why does the needle point N. and S. ? What is a 
dipping-needle ? Explain. How is a needle balanced ? 

268. Where is the N. magnetic pole ? How could one know 
when he reaches it? Does the earth induce magnetism? 
Which end of an upright bar will be the S. pole ? How has 
the lodestone become polarized? 

269. Define frictional electricity. The electroscope. Dif- 


QUESTIONS IN PHILOSOPHY. 


33? 


ference between static and dynamic electricity. Show the 
existence of two kinds of electricity. Give the names applied 
to each. 

271. State the law. What is the theory of electricity? Is 
it a polar force ? Is it easily disturbed ? Define a conductor. 
An insulator. 

272-3. What is the best conductor ? Best insulator? Isa 
poor conductor a good insulator ? When is a body said to be 
insulated? Can electricity be collected from an iron rod? 
Describe an electrical-machine. What is the use of the chain 
in the negative pole ? 

274. Define electrical induction. 

275. Faraday’s theory. 

276. Describe the electric chime. Explain. 

277. The dancing images. The Leyden jar. What gives 
the color to the spark ? 

278. How is the jar discharged ? What are the essentials of a 
Leyden jar? What is the object of the glass? The tin-foil? 

279. Give the theory of the charging of the jar. Can an 
insulated jar be charged ? Is the electricity on the surface or 
in the glass? Can the inner molecules of a solid conductor 
be charged? Will a rod contain any more electricity than a 
tube ? Why is the prime conductor of an electrical-machine 
hollow ? What is the effect of points ? How can we test this ? 

280. Describe the electric whirl. Explain the existence of 
electricity in the atmosphere. 

281. What is the cause of lightning? Thunder? Is there 
any danger when you once hear the report ? Describe the 
different kinds of lightning. Tell how Franklin discovered 
the identity of lightning and frictional electricity. 

282. What is the cause of the Aurora Borealis? How is 
this shown ? Prove the intimate relation between the aurora 
and magnetism. What are Geissler’s tubes? Gassiot’s cas¬ 
cade ? 

283. Tell what you can about lightning-rods. 

284. In what consists the main value of the rod? Does 
the lightning ever pass upward from the earth? A ns. It 
does, both quietly and by sudden discharge. Has Nature pro¬ 
vided any lightning-rods ? What is St. Elmo’s fire ? What 
is the velocity of electricity ? 

285-6. Illustrate its instantaneousness. Name some of the 
effects of frictional electricity—(1) Physical, (2) Chemical, 
(3) Physiological. 

287. How are galvanic electricity and chemistry related? 


QUESTIONS IN PHILOSOPHY. 


358 

Why is galvanic or voltaic electricity thus named? Tell the 
story of Galvani’s discovery. What was his theory ? 

288. Give an account of Volta’s discovery* What was his 
theory? How can we form a simple pile? Describe the 
simple galvanic circuit. 

289. Define the poles. Electrodes. Closing and breaking 
the circuit. What is necessary to form a voltaic pair ? 

290. Are the terms applied to the metals the same as those 
to the poles? Give the chemical change. Why does the 
hydrogen come off from the copper ? 

291. Tell what you can about the current. What really 
passes along the wire ? How is this force transmitted ? Will 
a tube, then, convey as much electricity as a rod ? 

292. Describe Smee’s battery. Grove’s battery. The 
chemical change. 

293. The advantages of Grove’s battery. Describe Bun¬ 
sen’s battery. Daniell’s battery. 

294. Sulphate of copper battery. Define quantity and in¬ 
tensity. Upon what do they depend? Compare frictional 
and galvanic electricity. 

295. Give the effects of galvanic electricity, (1) Physical— 
heat and light; (2) Chemical—decomposition of water, elec- 
trolosis, electrotyping, duplicates of copper-plates, and elec¬ 
tro-plating; (3) Physiological. 

302-3. What is the effect of a voltaic current on a magnetic 
needle ? What is a galvanometer ? 

304. An astatic needle ? An electro-magnet ? A helix ? 

305. Show how a helix can be magnetized. How are bar- 
magnets made ? How is motion produced by electricity ? 
Describe Page’s rotating-machine. 

306. What is the principle of an electric engine ? What 
difficulty is there in the way of its practical use ? Describe 
the magnetic telegraph. 

307. How is a message sent? How is one received? 

308-9. What is a sounder? What is the general principle of 

the telegraph ? Describe the relay. Name the use of each 
instrument. 

310-11. Define magnetic electricity. Describe a magneto¬ 
electric machine. Describe Wilde’s machine. Induced cur 
rents. 

312. Ruhmkorft’s coil. 

313. Thermal electricity. A thermo-electric pile. 

314. Describe the electric fish. 


INDEX 


Acoustics. 151 

41 figures... 173 

Action and Reaction... 78 

Adhesion. 40 

Air. 132 

Air-pump. 132 

Alcoholmeter. 118 

Amplitude. 58 

Aniraalculae. 20 

Annealing. 39 

Arm, The. 89 

Artesian Wells. 112 

Atmosphere. 132 

Atomic Theory. 19 

Attraction. 33 

41 nf AHhftsinn 40 


Cohesion. 36 

Capillary.,.. 41 
Gravitation. 48 


Aurora. ... 282 

Barker’s Mill. 127 

Barometer.. .. 141 

Battery, Bunsen’s. 293 

44 Grove’s. 292 

44 Sulphate of 

Copper. 293 

Thermo-e 1 e c - 

trie. 313 

Bell. 175 

Boiling.238, 244 

Brittleness. 32 


Camera. 220 

Capillarity . 41 

Capstan. 91 

Cartesian Diver. 134 

Centre of Gravity. 54 

44 Oscillation... 60 

Chemical Affinity. 35 

Chromatic Aberration.. 215 

Clock. .61, 64 

Clouds. 251 

Cohesion. 36 

Coils, Induction. 312 

Color. 209 

44 -blindness. 223 

44 Prismatic. 204 

44 Complementary.. 206 

Compass. 266 

Compensation Pendu¬ 
lum . 62 

Compressibility. 26 

Conductois. 271 

Cords. 169 

Correlation of Forces.. 316 

Crystals.38, 315 

Current, Voltaic. 291 

44 of rivers. 124 

Curves, Magnetic. 263 


Declination 
Density.... 
Dew........ 


266 

32 

250 


Diamond Jar. 286 

Diathermancy. 227 

Diffraction. 210 

Diffusion of Liquids.... 44 

44 Gases. 45 

Distillation. 237 

Divisibility. 17 

Ductility. 27 

Ear, The. 178 

Ear-trumpet. 159 

Echoes. 161 

Elasticity. 29 

Electric Battery. 292 

44 Light. 296 

44 Telegraph. 306 

44 Whirl. 280 

Electrical-machine. 273 

Electricity. 259 

44 Frictional... 269 

44 Galvanic.... 287 

44 Magnetic. .. 261 

Electrodes. 289 

Electro-gilding. 300 

44 -magnetism. 302 

44 -magnets. 304 

44 -negative and 

positive sub¬ 
stances. 298 

44 -plating. 300 

Electrolysis. 298 

Electrolyte. 298 

Electroscope. 269 

Equilibrium. 54 

Evaporation. 241 

Expansion. 233 

Eye, The. 220 


Falling Bodies. 

Far-sightedness. 

Filtering.. 

Fire-engine. 

Flames, Sensitive. 

44 Singing. 

Floating Bodies. 

Focus. 

Fogs... 

Force. 

44 Pump. 

44 Centrifugal. 

44 Centripetal. 

44 Composition of... 

44 Molecular. 

44 Resolution of.. . 

Fountains. 

Freezing Mixture. 

44 of Water. 

Friction. . 


50 

222 

23 

145 

182 

182 

119 

200 

250 

67 

144 

75 

75 

72 
35 

73 
112 
233 
256 

67 


Galvanometer. 303 

Gases. 227 

44 Adhesion of. 44 

44 Buoyancy of. 137 

44 Compressibility. .26, 30 


Gases, Diffusion of. 4 » 

44 Elasticity of..... 29 

44 Osmose of.. 46 

44 Pressure of. 132 

Gassiot’s Cascade. 283 

Geissler’s Tubes. 283 

Gold Leaf. 29 

Governor, The. 247 

Gravitation. 48 

Gravity. 49 

44 Centre of.. 54 

44 Specific. 115 

Gulf Stream. 254 

Ilalos. 214 

Hardness. 31 

Harmonics. 175 

Ileat... 225 

44 affected by Rare¬ 
faction . 248 

44 Absorption of.... 246 
44 Conduction of.... 243 

44 Convection of.... 245 
44 Expansion by.... 233 

44 Latent. 232 

4 * Luminous. . 227 

44 Mechanical Equiv. 231 

4 * Quality of. 229 

44 Radiation of. 245 

44 Reflection of. 246 

44 Refraction of. 228 

u Solar. 230 

44 Specific. 230 

44 Theory of.. 228 

44 Vaporization. 237 

Heating by Steam. 241 

Helix. 304 

Horse-power, A. 100 

Hydraulics. 122 

Hydrometer. 118 

Hydrostatics. 103 

11 ydrostatic Bellows.... 110 

44 Paradox... Ill 
44 Press. 106 

Iceland Spar. 211 

Inclined Plane. 93 

Indestructibility. 26 

Induction. 264 , 311 

Inertia. 25 

Insulators. 271 

Isochronous. 58 

Joule’s Law. 232 

Kite. 75 

Lenses... 200 

Land and Sea Breeze... 253 

Lever. 85 

Lej r den Jar. 277 

Light. 185 

44 Composition of... 204 
44 Diffraction of.... 210 























































































































































34 ° 


INDEX. 



Light, Interference of.. 

207 

Northern Lights. 

282 

“ Laws of. 

183 



“ Polarized. 

210 

Oceanic Currents. 

254 

“ Reflection of.... 

190 

Octave. 

16 b 

“ Refraction of... 

197 

Opera-glass. 

218 

“ Theory of. 

189 

Optics.. . 

185 

“ Total Reflection 

197 

Optical Instruments.... 

215 

“ Velocity of. .... 

189 

Organ Pipes. 

177 

“ Waves of.. 

208 

Oscillation, Centre of... 

60 

Lightning. 

281 

Osmose of Gases. 

46 

Liquids, Buoyancy of... 

116 

“ Liquids. 

45 

“ Cohesion of.... 

36 

Overtones. 

175 

“ Compressibility 




of. 26 , 29 

Page’s Rotating Ma- 


“ Diffusion of.... 

44 

chine. 

305 

“ Elasticity of... 

29 

Pendulum. 

58 

“ Osmose of. 

45 

Perpetual Motion. 

79 

“ Pressure of_ 

114 

Pisa, Tower of.. 

56 

“ Specific Gravi- 


Pitch... 

163 

ty of.. 

118 

Platinum Wire. 

28 

“ tend to spheres 

37 

Pneumatics. 

132 

Liquefaction. 

236 

Pneumatic Inkstand... 

147 



Polarization of Light... 

210 

Machinery. . 

85 

“ Heat... 

228 

Magdeburg Hemisphere 

136 

“ Electric- 


Magic Lantern. 

219 

ity... 

264 

Magnetic Curves. 

263 

Porosity. 

21 

Magnetism. 

261 

Pressure of Air. 

132 

Magneto-electricity._ 

261 

“ Gases, f_ 

132 

Magnets. 

261 

“ Liquids. 

103 

Magnitude. 

25 

Prince Rupert Drop.... 

40 

Malleability. 

28 

Prisms. 

199 

Mariotte’s JLaw. 

141 

Pulley. 

97 

Measures, Standards of. 

15 

Pumps. 

142 

Mechanical Powers... . 

83 

“ Air. 

132 

Mechanics, Principle of. 

85 



Microscopes. 

215 

Rain. 

252 

Mirage. 

202 

Rainbow. 

212 

Mirrors. 

191 

Reaction. 

78 

Molecules. 

21 

Reflected Motion. 

79 

Molecular Forces. 

35 

Relay. 

309 

Momentum. 

68 

Resonance. 

161 

Motion. 

65 

Rest. 

67 

“ Compound. 

71 

Ruhmkorff’s Coil. 

312 

“ Circular. 

75 

Rupert Drop. 

40 

“ in a Curve. 

79 



“ Laws of. 

70 

St. Elmo’s Fire. 

284 

“ Perpetual. 

79 

Screw. 

95 

“ Reflection of.... 

79 

Sensitive Flame. 

182 

“ Resistance to... 

67 

Ship, Sailing of.. 

74 

Music. 

163 

Singing Flames. 

182 

Musical Scale. 

176 

Siphon. 

145 



Siren. 

164 

Near-sightedness. 

222 

Snow. 

253 

Needle, Astatic. 

303 

Solution. 

44 

“ Magnetic. 

263 

Sonometer. 

170 

“ Dipping. 

267 

Sound. 

151 

Newton’s Rings. 

207 

“ Intensity of. 

157 

Nodal Lines. 

173 

“ in a Vacuum.. .. 

155 

Nodes. 

171 

“ Interference of.. 

169 

Noise... 

163 

“ Reflection of..... 

160 


MO 

v_ 0ru.o\O 

l 


, E) 
0 


Cv 





6 


«C 


(HMA*- 


z 


/ 




Sound, Refraction of... 
“ Superposition of 

“ Velocity of. 

Sounding-boards. 

Sound Waves. 

Speaking Tubes. 

“ Trumpet. 

Specific Gravity. 

“ “ Flask.. 

Spectrum, Solar. 

Spherical Aberration... 

Spheroidal State. 

Steam. 

“ -engine. 

Steelyard. 

Stereoscope . 

Stringed Instruments .. 

Tacking. 

Telegraph. 

Telescope.. 

Tempering... 

Tenacity. 

Thermo-electricity. 

Thermometers. 

Thunder. 

Torsion Balance. 

Tourmaline. 

Trade-wind. 

Turbine Wheel. 


Velocity. 

Vibrations of Air. 

“ Cords. 

“ Ether.... 

“ Pendulum 

“ Solids .... 

“ Sympathetic 

Visual Angle. 

Voltaic Arch. 

“ Battery. 

“ Electricity. 

“ Pair, The. 


Water. 

“ -barometer.. 

“ -level. 

“ -wheels. 

Waves. 

Wave Motion. 

Wedge. 

W elding. 

Weight.. 

Wells. 

Wheel and Axle... 

Wheel work. 

Wilde’s Machine.. 

Winds. 

Wind Instruments 


156 

163 

155 

175 

152 

158 

159 
115 
118 
204 
214 
242 
241 
246 

88 

219 

170 


74 

306 

216 

39 

29 

313 

236 

281 

31 

211 

254 

126 


67 

152 

169 

189 

58 

228 

178 

188 

296 

292 

287 

289 


255 

142 

115 

125 

128 

129 

96 

36 

49 

112 

90 

92 

311 

253 

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